Ulum al Qur'an
Summary
Nothing can excel the tafsir of the Qur'an by the Qur'an. This is followed by sound reports about the Prophet's explanation of the revelation.
Whatever is sound and genuine in the explanation of the Qur'an by the sahaba and the tabi’un may not be rejected, but the following principles are to be observed:
Sound reports must be distinguished from unsound ones, for many views have been falsely attributed to some sahaba and tabi’un (especially to Ibn 'Abbas and Mujahid, the most renowned ones among them), which cannot be traced back to them when the isnad is investigated. Those reports must of course be rejected.
Material from the ahl-al-kitab, in particular the Jewish traditions (isra’iliyat) [For details, see below, p. 133.] must be sorted out and evaluated.
Material which crept in due to theological, philosophical, political and other considerations, must be sorted out and evaluated (such as e.g. some Shi'a attributions to 'Ali, or 'Abbasid attributions to Ibn 'Abbas, etc.).