Reason and unreason
Yet although Islam did not accept poetry, it could not remain unaffected by it. Poetry entered Islamic countries through lamentations and passion plays, although it was never able to attain an exalted literary position.
In many of the Qur'anic verses God invites humans to reason, to think and to find the path of righteousness. But "reason" is here limited to what is given or known in the Qur'an. Qur'anic rationality cannot go beyond the Qur'an, where it becomes kufr [unbelief]. The Qur'anic reason is a tool for arriving at belief and serving God. Qur'anic rationality is the affirmation of "there is no other God but Allah, and Mohammad is his Prophet". The whole of the book is written to prove this thesis. Under such rationality the proof of a subject or a phenomenon is not in what is provided by science but in the sayings of Muhammad and Allah.
Anything but what Islam and the holy book allow were banned. Religious bigotry grew and Muslims came to the conclusion that all branches of learning can be found within the Qur'an. Nothing else it is needed or permissible. This was the start of limitations to science and knowledge and book burnings.
In Islam a scholar and scientist  is one who has mastery of the Qur'anic scholarship. That is why a cleric in the Islamic religion is known as a religious scholar. The science of religion is the scholarship [elm] of religious jurisdiction (feqh) and a religious jurisprudence (faqih) is one who has mastery of Islamic commandments. The faqihs recognise the science of religion as the sole authentic science in the world, because the real salvation of humans, both in this and the afterlife, lies within it.
In Islam science, like history, is something that is "given", unchanging. Linked to the will of Allah. There is no place there for the humans and human intelligence. Everything revolves round destiny. Religious science in Islam is divided into various branches: the science of hadith [sayings], science of interpretation, the word, feqh, osul etc… Pre-Islamic Arab poetry was outside this stockade and could not continue to live within its framework. Poetry and poets were travelling a road against Islamic wisdom and rationality: "as the bedouin said, the story of life, death, and a return again to life is nothing but a fable". 
The Arabs considered poetry (butiqa) as part of the nine realms of knowledge which were the precursors of all the rational (aghlieh) and intellectual (zehnieh) sciences. But the Arab of "the early Islam and a large part of the period of the Umayyad caliphs (661-750 AD) showed absolutely no interest in the 'rational' sciences and considered the book of Allah and the traditions (sunnah) of the Prophet sufficient for happiness". Muslim Arabs in the early Islamic period paid no attention to anything other than the laws of Islam - the Qur'an, destroying whole libraries in the wake of their conquests. 
Pre-Islamic Arab literature was mainly oral consisting of the reciting and compilation of poetry. Narration was more important than writing. There were the rawi, who memorised and recited poems. Poetry reading sessions had a special importance. With the establishment of Arab rule Arab poetry, and alongside it Parsi poetry, was transformed. Arab poetry and literature came into contact with the rich Pahlavi, Greek, Indian and Syriac literature. With the spread of Islam many of these works were translated into Arabic, and enriched that language. The Arabs became familiar with other cultures and Islamic culture blossomed. Arabic slowly became the scientific language of its era and this influenced Farsi. The Iranians, therefore, owe the developments of the Farsi language to Arabic.
The Arabic tongue, simply by being the language of the Qur'an, became the holy language of Muslims. This sanctity persists today despite the numerous translations of the Qur'an into Farsi. Although many Iranians do not know Arabic they still consider the reading of the Qur'an in Arabic as a pious deed. The reciting of the Qur'an is indeed one of the arts and sciences of Islam. Through this Arabic jeopardised any national or cultural independence of the countries under Arab rule. The language of religion was that of government and the language of government was alien to the language of the people of Iran. Thus the Iranians had to make their position clear.
The Muslims divided worldly knowledge and information into rational and traditional (historical) sciences. The latter was subdivided into religious and literature.
Ibn Khaldun (1332-1406 AD) in his Introduction divides human sciences into two: "philosophical, or wisdom sciences which accrue to mankind qua thinking being, and which are accessible to the intelligence and native to the nature of human thought. Distinct from this group is that which assembled the sciences based upon the absolute truth of certain narrative texts (akhbar and usul). These are the traditional positive sciences which unlike the universally uniform wisdom sciences, are specific to each religious community. The positivity of these sciences resides in their fundamental procedure which is the employment of scared text as the repository of truth and the validity of situations subsequent to it on the basis of the text … made up of the Qur'an and tradition" . Further down Ibn Khaldun refutes marginal sciences (magic, letters and alchemy) and philosophy thoroughly as "they [philosophy] are very common and extremely harmful to religion and civilisation" 
Thus what Islam calls the science of "adabyeh" (literature) cannot exist outside the Qur'an. Whatever is considered science (knowledge) for Muslims must in one way or another be in harmony with the Qur'an.
Poetry, alongside other non-Qur'anic knowledge, is superfluous. There is a hadith from the Seventh Imam  "when the Prophet of Allah entered a mosque, he saw people gathered around an individual. He asked: what is happening? They replied he is a learned man. He asked what is a learned man? They replied one who wisest to the ancestors of Arabs, their happenings, the jahelyya (pre-Islamic) times, and Arabic poetry. The Prophet said: these are sciences that not knowing them will do not harm and of no value to the scholar. He then added, scholarship is three things: Qur'anic verse, religious duty [farizeh adeleh], and established sunna (tradition). And in addition is learning." 
Thinking in Islam is confined to the Qur'anic verses, and the aim of reason (aql) is to understand the word of God for the salvation of humanity. Therefore aql is a tool for religion and its protector . Thus the science of reading, interpretation (tafsir), jurisprudence (feqh), traditions (hadith), the word (kalam) etc are all linked in one way or another to the Qur'an - hence the importance of reading the holy book. The reader of the Qur'an also memorised it and passed this on to the next generation.
Even though the Iranians accepted Islam through the sword, whenever given the chance, they interfered and amended it. They even came to the idea to change the official language of worship form Arabic to Farsi. Abu Hanifeh  was one of those who advocated this route. Given the long history and strong base of poetry in the pre-Islamic Arabic as well as the Iranian languages Islam was unable to bury it.
From the 10th Century, the Arabic influence on Farsi spread, the two became intermingled and new words of Arabic roots entered Farsi. New expressions also appeared. Some were devoted to Islam such as quessas [an eye for an eye], khoms, zakat [forms of religious taxation]. Some such as adel (just), emir, hakim (ruler) etc had bureaucratic usage.
There was a reciprocal relationship with Farsi words, especially those relating to sciences and the running of the country, entered Arabic. Indeed the Qur'an contains many words taken from Farsi including "terms that relate to paradise and its pleasures. This included the very term ferdows (paradise) which is from Avesta" . The Arabs knew of the splendours and glory of Iranian court life and, compared to Arabia, Iran was a developed and mystical dreamland that was truly paradisical.
With increasing influence and spread of Islam in Iran, the influence of Arabic on Farsi increased further. Arabic entered Iranian poetry and writing. But there were geographic factors which influenced this intermingling. During the Samanid dynasty (819-1005) whose capital in Khorasan was far from Baghdad, Mecca and Medina, the influence of Arabic on Farsi was less.
Mohamad bin-Vasif, writer of dissertations to Ya'qub Saffari (Saffarid dynasty 867c-1495) is said to be the first to write poetry in Farsi. The story goes that Ya'qub who was being eulogised in Arabic one day said to his poets: "why recite something that I cannot understand?"  The writer may have meant official court poetry since Farsi poetry had been written from times past. 
What is clear is that at this time Farsi poetry was written with Arabic meter, and provided it did not conflict with the spirit of Islamic teaching, it flourished. From now on the soul of religion was to dominate the body of the verse. Whenever, and to the extent that poets were able to free themselves from this spirit, they also distanced themselves from the influence of Arabic. Conversely whenever the poet was severely influenced by Islam and the shari'a, then Arabic words entered the verse in large amounts and the poem itself was caged by the limitations of Islamic philosophy. "Since Farsi poetry was created on the basis of Arabic poetry, it often modelled itself on Arabic poetry in it expressive moulds and the motives". 
In the tenth century attention turned to innovation, meter and rhyme and books were written on them. All the same "the Farsi poem even if it takes its prosodic meter totally from Arabic poetry, follows the musical taste and sound of its own language and realm, and thus retains its own peculiarities even in its prosodic meter" . But in these years poetry takes second place to translation and history. With Islam standing over them poetry and poets for a time lost their lustre.
From the 11th and 12th Century the poet took a broader look at the world. Philosophy and ethics entered the world of verse, and no poet was seen to be great and important if they stayed away from the sciences of the time. Nezami Oruzi in his Four Articles wrote "…the poet does not reach this level and deserve the designation of a master unless in the prime of life and in their youth they learn twenty thousand stanzas of their predecessors, and have in front of their eyes ten thousand words from their contemporaries and continuously read the collections of masters and learn them….and under a renowned master study the analysis of meaning and words and translations and all such sciences…". In those years you could not claim to be a poet without the aid of other sciences. It is in this "progress" that poetry flourished. Couplet poems (mathnavi), elegy (qasideh), lyric poem (qazal), quatrain (rubayi) and strophe-poem (tarjih band) etc enter verse form.
In these years, and especially in the ninth century, a new development took place in Farsi poetry with the introduction of erfan (mysticism) and sufism. Poets slowly distanced themselves from the court and the caliphs.