The word “sources of Tafsīr” does not mean the disciplines the mufassirun used to rely on when explaining the Qur’ān according to the idea they carried such as tawheed, jurisprudence (fiqh), rhetorics (balaagha), history etc. These are not the sources of tafsīr. Rather, they are matters which had an effect on the mufassir which led him to lean towards a specific matter in tafsīr. What is intended by 'sources of tafsīr' are the authoritative sources that the mufassirun qouted. What they quoted they wrote down in their tafsīrs, irrespective of their orientation in tafsīr. If we study the sources of tafsīr we find that they are confined to three:

First: Tafsīr which has been transmitted from the Messenger of Allah  such as the narration that the Messenger  said:

For instance what has been narrated by 'Ali that he said:

And what has been reported ;

However this genre of tafsīr cannot be relied upon as a source of transmission save what has been reported in the books of Sihah (books of hadīth such as Bukhari and Muslim) because the storytellers and fabricators greatly added to the material. That is why this type of source material has to be investigated due to the great number of lies against the Messenger of Allah . The scrutiny of the Salaf (early generations of scholars) of this genre of tafsīr reached the point where many of them rejected the whole genre altogether...They held that no tafsīr had been transmitted from the Messenger of Allah . It has been reported about Ahmad bin Hanbal that he said:

That is why we find the mufassirun due to their lack of trust for what is mentioned did not stop at the limit of what has been reported. Rather, they followed that with what they reached through their own Ijtihād. They did not stop at the limit of the text. What has been mentioned about the Sahabah in terms of tafsīr was added to the ahadīth about the Messenger of Allah . It began with the transmitted tafsīr and likewise with the tafsīr of the tabi'un. This genre of transmitted tafsīr became huge and it began to include what has been transmitted about the Messenger of Allah , the Sahabah and Tabi'un. Each one came to suffice as a tafsīr. Nearly all the books of tafsīr written in the early ages were restricted to this manner of tafsīr.

Second: One of the sources of tafsīr is the opinion (ra'y) or what is termed as Ijtihād in tafsīr. This is because the mufassir knows the speech of the Arabs and their mannerisms in the spoken language. He knows the Arabic expressions and their meanings by being acquainted with the same thing in jahili poetry, prose etc. And he familiarises himself with what he finds to be correct from the cause of revelation of a verse. Using these tools he explains the Qur’ānic verses in accordance with what he has reached through his Ijtihād. Tafsīr by opinion does not mean saying whatever one likes about the verse or whatever our own desires demand. Rather, the opinion according to which the tafsīr takes place depends on the jahili literature in terms of poetry, prose, the customs and speech of the Arabs. At the same time it relies on the events that took place in the days of the Messenger  and whatever the Prophet  faced in terms of hostility, conflicts, migration (hijra), wars and afflictions. And whatever happened during that period which required hukms and demanded the revelation of the Qur’ān. Therefore, what is meant by performing tafsīr by opinion is to understand the sentences by understanding its meanings which are indicated by the information the mufassir has at his disposal in terms of the language and events. As for what has been narrated about sayyidina 'Ali ibn Abi Talib that he said:

This does not mean that the Qur’ān is open to any viewpoint you wish to explain it from. Rather, what is meant is that any one expression or sentence is open to a number of viewpoints in tafsīr but the viewpoints are restricted to the meanings the expression or sentence is open to, which does not overstep that limit. Consequently, tafsīr by opinion means the understanding of a sentence within the limits of the meanings its expressions are open to. That is why they called it tafsīr by Ijtihād.

The great majority of the mufassirun from the Sahabah used to explain the Qur’ān by opinion and rely on it in the first degree while performing tafsīr. They used to disagree in tafsīr even in the explanation of a single word which indicates that they used to rely on their own particular understanding like much of what has been reported about Ibn 'Abbas, Ibn Mas'ud, Mujahid and others. For example they used to explain the word Tur in His  saying:

with different explanations. Mujahid explained Tur as the name of a mountain, Ibn 'Abbas explained it as the mountain itself and some said that Tur is what emanates from the mountain and as for what does not spread, it is not Tur. This difference in tafsīr is a result of a difference of opinion and not attributable to the difference in what has been transmitted. Although the expression is linguistic so what about when the opinion concerns the meaning of the sentence and not the meaning of an expression, this is why in addition to their disagreement in the meanings of expressions they also disagreed with regards to the meanings of verses. It is apparent from studying the tafsīrs of the Sahabah especially the well known mufassirin that on a whole they would rely on (individual) opinion when making tafsīr. As for what has been narrated about some of them that they would refrain from making tafsīr by opinion and confined themselves to making tafsīr by what has been transmitted (manqul), it is taken to refer to the opinion of somebody who has not acquired the tools of tafsīr such as having knowledge of the Arabic expression, he wishes to clarify and knowledge of the events pertaining to which the verses were revealed. It is not taken to mean that one should refrain from understanding the Qur’ān since it has been revealed so that people may understand it and not so that they restrict themselves to the limit of the transmitted tafsīr. Therefore, we cannot say that the Sahabah were divided into two groups, one group refraining from saying anything about the Qur’ān by opinion and the other speaking about the Qur’ān according to their opinion. Rather, all of them used to speak about the Qur’ān with their opinion. They used to be wary of someone saying anything about the Qur’ān by his opinion without having sure knowledge of the expression being explained and the sentence being elucidated from the ayāt of the Qur’ān. Also, this was the position of the Tabi'un. However, after them there were people who came to know about these sayings and understood them as admonition for speaking about the Qur’ān with one’s own opinion, so they avoided saying anything about the Qur’ān from their own opinion. And there were also people who became acquainted with the the tafsīr of the Sahabah by opinion and they advocated tafsīr by opinion. That is why later scholars became divided into two groups regarding the tafsīr. One group would avoid saying anything with their own opinion and restrict themselves to what has been transmitted and a group that would give its own opinions. As for the Sahabah and the Tabi'un they were not two groups. Rather, they used to speak about the Qur’ān with what they knew in terms of the narrations and opinion and they refrained from that which they did not know and they warned people from speaking about the Qur’ān with their own opinions without having knowledge.

Third: The israiliyyat. This is because certain Jews and Christians had entered the fold of Islam. Among them were scholars of the Torah and Bible. Amongst them most of the Jews that entered were dishonest because the Jews hated and loathed the Muslims more than the Christians. From the Jews many scholars of Jewish fables infiltrated the Muslims. They entered the tafsīr of the Qur’ān to supplement the explanation of the Qur’ān. That is because the mind and its inclinations are fond of inquiry which invited it when listening to many verses of the Qur’ān to inquire about things surrounding them. When they heard the story about the dog and the companions of the cave they asked what colour was the dog? When they heard:

They inquired as to what was that piece with which they struck the dead man? When they read:

They asked us, who is the righteous servant that Musa met and requested to teach him. From here the story of Khidr arose. So whenever a story would reach them they would ask about it. So they asked about the boy the righteous servant had killed and the boat he had scuttled and about the village that did not entertain him. They inquired about the story of Musa and Shu'ayb and the size of Noah's ark etc. What answered these questions and satiated their greed for this kind of information was the Torah and its commentaries and exposition. And whatever fables were inserted which were transmitted to them by the Jews whether through good or bad intention, some Christians who had embraced Islam inserted certain stories and reports from the Bible but that was little compared to what the Jews had interpolated. In this manner the volume of stories and reports expanded greatly until it exceeded the reports of the transmitted tafsīr. Many books of tafsīr came to be loaded with huge amounts of israilyyat, stories and other reports. The ones who inserted the greatest amount of israilyyat and the most famous were Ka'b al-Ahbar, Wahb ibn Munabbih, 'Abd Allah ibn Salam and many others. Due to this activity these israilyyyat, stories and other reports became one of the sources of tafsīr for the mufasssrun.