The clear beliefs, irrefutable rules, instructions and messages directly mentioned in muhkam verses of the Holy Quran are parts of Islam proved by tawatur and tawaruth. This part is called 'Ummul Kitab'. The path described in the Ummul Kitab is 'Sabilul Mu'minin' in the context of the Quran. They are the foundations of Iman and Islam. They do not need any interpretation. They are very clear, one can understand them from the recitation of the Quran. Moreover, they are proven with tawatur and tawaruth from the Prophet sallallahu alaihi wasallam. For these reasons, this Muhkam part of the Qur'an, which is very wide, is itself a major source of tafseer. Regarding this part of the Quran, it is said in usulud din,

تأويله تنزيله

Apart from this part, there is another part of the Holy Qur'an, to understand which we need tafsir because of the limit of our understanding. There are several sources for this tafsir. The third of them is Athar. Israili narrations are not in this list. Rather, it is considered as an unreliable source of Tafsir. Students who have read at least Ulumul Quran by Mawlana Muhammad Taqi Usmani Damat Baraktuhum also know this.

Athar means the statements of the Sahaba (Athar e Sahaba), the statements of the Tabi'iin and the statements Imams of Din (مقاطيع). All of them are included in the sources of Tafsir of the Quran with differences in their status.

Only experts of Ulumul Hadith, Usulut Tafsir, Usulul Fiqh and Usulud Din have ability to understand and assess the acceptability of a Tafsir, as well as to determine the position of of that tafsir.

 A few days ago a student of higher education told me, I have done Muraja'ah in two places of ad-dur al-mansur by Jalaluddin Suyuti r.a.. I am very upset after seeing some athar from it. When I read the footnotes printed with Jalalain with big size in our Indian subcontinent, sometimes I get very confused. Some strange things have been narrated from Sahaba and Tabi'iin. I don't understand how sahabah and tabi'iin could say this kind of baseless things!

He was advised, first keep in mind that our understanding is not a criteria of some narration being right or wrong. The narration about which experts say right in accordance with criterion of shariah is right and about which they say wrong is wrong.

The second thing is that you may have thought- everything narrated from the Sahaba-Tabi'iin in any book of Tafsir is proven from them- this thought is not correct. The reason for this is that, first of all, we have to see, how is the sanad of this saying attributed to the Sahabi or Tabi'i? The footnote of Jalaline usually does not have any reference, nor does it have a sanad. Ad-durr al-mansur usually has reference but in most places Sanad is not mentioned.

It is necessary to verify the quality of the chain of authority of the narration from the book mentioned in reference or any other basanad book . Obviously, this work can only be done by the experts of Ulumul Hadith. Our duty is to learn from them and take their decisions.

If you check the quality of traditions in this way, you will find that most of your questions will disappear automatically. After that you will have another work- you have to know the correct meaning of the traditions that has been accepted through verification. It is the work of experts in usul at-tafseer and usul al-fiqh. This work cannot be done only with the knowledge of Arabic language. Just as it is necessary to have sufficient knowledge of Arabic language to know the correct meaning, it is also necessary to have expertise in other related subjects so that one may be able to understand to which word or sentence of the verse this athar is related or to which part of tafsir the athar is connected. Then he may be able to understand the source of this athar.

I said the subject in a few lines. But experts know how much effort, ability, expertise is needed for this work. 

In this stage too, our work is to follow the correct thing after knowing it from the experts. 

If you do this work all of your questions will be answered in sha allah. This is why it is warned again and again that only Arabic language is not enough for tafsir. It is also warned that knowledge dependant on translation is dangerous. It is not safe without instructions and supervision of the Ulama. 

There is another sensitive issue, to which hadhratul ustadh Mufti Muhammad Taqi Usmani pointed out. He mentioned israili tafsir in the list of unreliable sources of tafsir.  Then he said after discussing types of israili narrations and their status elaborately, 

بعض روایات میں تو صراحت ہوتی ہے کہ یہ اسرائیلی روایت ہے،  اور بعض روایات میں ایسی صراحت نہیں ہوتی، لیکن دوسرے دلائل کی روشنی میں معلوم ہو جاتا ہے کہ یہ اسرائیلیات میں سے ہے، تفسیر کی کتابوں میں جو روایات کعب الاحبار اور وھب بن منبہ سے مروی ہیں  وہ زیادہ تر اسی قسم سے تعلق رکھتی ہیں۔

 Sometime it is mentioned in the narration that it is israeli narration. Sometimes it is not mentioned in the narration. But in the light of other narrations it is known that it is also israeli narration. Most of the narrations narrated by Kab al-Ahbar and Wahab ibn Munabbih in the books of tafsir are like this. 

قال عبد المالك: فمن لم يتنبه لذلك من الطلاب يقول في هذه الإسرائيليات أنها من أقوال التابعين، فإن كعبا ووهبا  كلاهما من التابعين، ثم يدعي أن أقوال التابعين من مآخذ التفسير، وهو لا يدري أن هذه الأقوال ليست أقوال التابعين، وإنما هي حكاية منهما من أهل الكتاب، فهي من الإسرائيليات، والإسرائيليات ليست مأخذا للتفسير أبدا.

قال عبد المالك: وأدق من ذلك أن بعض الصحابة رضي الله تعالى عنهم قد يحكي عن أهل الكتاب بعض الأشياء إما استغرابا وإما استنكارا (وإما لأخذ العبرة إذا كانت قصة غير منكرة فيها عظة) فيظنه البعض بمجرد رؤية اسم الصحابي أنه قول الصحابي، وهو مأخذ للتفسير، مع أن المأخذ هو قول الصحابي، لا ما حكاه عن أهل الكتاب والإسرائيليات.

والتمييز بين أقوال الصحابة وبين حكاياتهم عن الإسرائيليات إذا لم يكن التصريح بالحكاية مذكورا في الرواية أمر غامض ومشكل لا يستطيع أن يقوم به إلا النقاد الفحول من أمثال ابن كثير والآلوسي وغيرهما.

لكن لا يأتي في أقوالهم في أمور الدين من العقائد والأحكام شيء من الإسرائيليات،فإنهم كانوا لا يأخذون ولا ينقلون شيئا عن اهل الكتاب في أمور الدين في قليل او جليل، وإنما يأتي النقل عنهم في القصص وأخبار الأنبياء والأمم السابقة،اعتمادا على الترخيص النبوي، فهناك ينبغي الانتباه حتى لا يعد النقل عن الإسرائيليات عين أقوال الصحابة والتابعين. ومن أراد المزيد من الشرح والتفصيل في هذه المسألة الشائكة يمكنه أن يراجع الوجيز في شيء من مصطلح الحديث ص৩৯-৪০১ ومحاضرات علوم الحديث ص১৭২-৩৭২، كلاهما للعبد الضعيف عفا الله تعالى عنه وعن والديه وشيوخه.

This subjects remind us of a quote of Imam Malik r.a. as he says, 

ليس في العلم شيئ خفيف

We hope that our students will always keep in mind the importance of ilm and try their best to be experts in ilm, tafqquh, tafattun and tayaqquz.