The Sources Of Tafsir (http://www.qtafsir.com/)
If someone asks about the best methods of Tafsir, we reply that the best method is to explain the Qur’an with the Qur’an itself. What is mentioned in general terms in one place in the Qur’an, is usually explained in another place. When one does not find this easily, he should look to the Sunnah because its purpose is to explain the Qur’an and elaborate upon its meanings. Allah said,
‘Surely, We have sent down to you (O Muhammad SAW)  the Book (this Quran) in truth that you might judge between men by that which Allah has shown you, so be not a pleader for the treacherous.’ (An Nisa 4:105)
‘And We have also sent down unto you (O Muhammad SAW) the reminder and the advice (the Quran), that you may explain clearly to men what is sent down to them, and that they may give thought.’ (An Nahl 16:44)
So one seeks the Tafsir of the Qur’an with the Qur’an itself and with the Sunnah. If one cannot find the Tafsir in the Qur’an or Sunnah, he should refer to the statements of the Companions, who were the most knowledgeable of Tafsir, for they witnessed the situations and incidents that we did not witness. They also had the deepest comprehension, the most correct knowledge, and the most righteous works. Especially the scholars and leaders among them, such as the Four Rightly Guided Khalifahs and righteous Imams, and `Abdullah bin Mas`ud, may Allah be pleased with them all. Imam Abu Ja’far bin Jarir At-Tabari narrated that ‘Abdullah bin Mas`ud said,
“By He other than Whom there is no God, no Ayah in the Book of Allah was revealed but I have knowledge about whom and where it was revealed. Verily, if I know of a person who has more knowledge than me in the Book of Allah that the animals can reach (by travelling on them), I will travel to meet him.”
Also, among the scholars of the Companions is the great scholar, the sea of knowledge, ‘Abdullah bin `Abbas, the cousin of the Messenger of Allah, and the explainer of the Qur’an, as a result of the blessing of the supplication of the Messenger of Allah peace be upon him. The Prophet invoked Allah for the benefit of Ibn `Abbas,
(O Allah! Teach him Fiqh in the religion and interpretation.) 
Further, Ibn Jarir At-Tabari reported that `Abdullah bin Mas`ud said, “Yes, Ibn ‘Abbas is the interpreter of the Qur’an.” This Hadith has an authentic chain of narrators.  Ibn Mas`ud died in the thirty-second years of Hijrah and ‘Abdullah bin `Abbas lived for thirty-six years after that. Hence, what do you think about the knowledge that Ibn ‘Abbas collected after Ibn Mas`ud? Al-A’mash said that Abu Wa’il said, “Ali once appointed ‘Abdullah bin `Abbas to lead the Haj season. Ibn ‘Abbas gave a speech to the people in which he read and explained Surat Al Baqarah (according to another narration, Surat An-Nur) in such a way, that if the Romans, Turks and the Daylam heard him, they would have embraced Islam.” 
 was given the Qur’an and its equal with it)), in reference to the Sunnah. 2
 Ahmad 4:131
 Fath Al-Bari 1:205.
 At-Tabari 1:90.
 At-Tabari 1:81.
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