Ulum al Qur'an
The Qur'an explained by the Qur'an
The interpretation of the Qur'an by the Qur'an is the highest source of tafsir. Many of the questions which may arise out of a certain passage of the Qur'an have their explanation in other parts of the very same book, and often there is no need to turn to any sources other than the word of Allah, which in itself contains tafsir. To seek to explain an aya from the Qur'an by referring to another aya from the Qur'an is the first and foremost duty of the mufassir. Only if this does not suffice, he will refer to other sources of tafsir. [Itqan, 11, pp.181-2.]
Examples:
A case in point is the detailed explanation of 5:2 by 5:4, concerning permissible and prohibited meat. Another example of explanation of one aya in the Qur'an by another concerns a question which might arise from Surah 44: 3. It is explained in Surah 97: 1:
'We sent it down during a blessed night' (44: 3).
Which night is this blessed night, in which the Qur'an was sent down?
'We have indeed revealed this in the lailat al-qadr' (97: 1).
A third example is the explanation of Surah 2:37 by Surah 7:23:
'Then learnt Adam from his Lord words of inspiration, and his Lord turned towards him, for He is Oft-Returning, Most Merciful' (2: 37).
These 'words of inspiration' are explained by the Qur'an as follows:
'Our Lord! We have wronged our own souls. If Thou forgive us not, and bestow not upon us Thy mercy, we shall certainly be lost' (7: 23).