God Himself confirms that the Qur'an has been preserved from change...

God Himself confirms that the Qur'an has been preserved from change; in chapter XV:9 He says, "Indeed We, even We, reveal the Reminder and indeed We are truly its guardian," and in chapter (XLI:41-42) He says, 'for indeed it is an unassailable Book. Falsehood cannot come to it from before or behind it. (It is) a revelation from the Wise, the Praise one." Only a divine Book could remain preserved for fourteen centuries in a world where the enemies of truth and of Islam are numerous.

* * * (Allamah Tabatabai, The Qur'an in Islam, p. 101-103) The Glorious Qur'an and exegesis (Tafsir) The Science of Qur'anic Commentary and the Different Groups of Commentators After the death of the Prophet a group of his companions, including Ubayy ibn Ka'b, 'Abd Allah ibn Mas'ud, Jabir ibn 'Abd Allah al-Ansarl, Abu Sa'ld al-Khudrl, 'Abd Allah ibn al-Zubayr, 'Abd Allah ibn 'Umar, Anas, Abu Hurayrah, Abu Musa, and, above all, the famous 'Abd Allah ibn 'Abbas, were occupied with the Science of Commentary.

Just as they had heard the Prophet explaining the meanings of the verses, they would transmit it orally to other trustworthy persons. The traditions specifically concerned with the subject of Qur'anic verses number over two hundred and forty; many were transmitted through weak chains of transmission and the texts of some have been rejected as incorrect or forged.

Sometimes the transmission would include commentaries based on personal judgments rather than on a narration of the actual sayings, hadiths, from the Prophet. The later Sunni commentators considered this kind of commentary as part of the body of Sayings of The Prophet, since the companions were learned in the science of Qur'anic commentary.

They argued that these companions had aquired their knowledge of this science from the Prophet himself and that it was unlikely they would say anything which they themselves had invented. There is, however, no absolute proof for their reasoning. A large proportion of these sayings, or traditions, about the reasons and historical circumstances of the revelation of verses do not possess an acceptable chain of narration.

It should be noted that many of the narrators like Ka'b al-Akhbar, were learned companions who had belonged to the Jewish faith before accepting Islam. Moreover, it should not be overlooked that Ibn 'Abbas usually expressed the meanings of verses in poetry.