What do traditional Jewish sources say about B'midbar Rabba regarding its date of redaction and authority vis-à-vis works of Chazal (such as other Midrashim).
Maharatz Chajez (in the beginning of part 2 of Kol Sifrei Maharatz Chajez, see pg. 530) has a piece inspecting the antiquity of various famous medrashim.
It is conveniently printed in the standard Vilna edition of the Medrash Rabba set as one of the last introductory sections.
His analysis, based on lack of being quoted in early Rishonim, leads him to say (on pg. 533) that Bemidbar Rabba was not as ancient as some of the other sections. He lays this claim against the section on Shmos and Devarim as well.
There is a rebuttal printed as a footnote to this introduction in the new Moznaim editions.
The source is Moriah year 22, gilyon ה-ו, ניסן תשנ׳ט. He cites two places where the Ramban quotes the Shmos Rabba.
He goes on to quote R' Moshe Greenes in his work on the Ramban Keren Pnei Moshe, that Shmos Rabba is quoted by Ramban 36 times, Bamidbar Rabba is quoted 12 times, and Dvarim Rabva 3. This led him to disagree with Maharatz Chajes.
However, he quotes Rashbam in Bava Basra 134a where he explains what the gemara means by Hagados as being 'Tanchuma, Bereishis Rabba, Vayikra Rabba' etc. This implies that Rashbam did not have Shmos Rabba.