(And they measure not the power of Allah its true measure when they say: Allah hath naught revealed unto a human being…) [6:91].
Al-Walibi reported that Ibn ‘Abbas said: “The Jews said to the Prophet: ‘O Muhammad, has Allah revealed a Scripture to you?’ The Prophet answered: ‘Yes, He did!’ They said: ‘By Allah, Allah has never sent down any Scripture from heaven’. Allah, exalted is He, then revealed (Say (unto the Jews who speak thus): Who revealed the Book which Moses brought, a light and guidance for mankind…)”.
Muhammad ibn Ka‘b al-Quradi said: “Allah commanded Muhammad, Allah bless him and give him peace, to ask the people of the Book about his matter and how he is mentioned in their Scriptures. However, their resentful envy toward Muhammad drove them to disbelieve in Allah and His Messenger. They said: ‘Allah has never revealed anything to any human being’. And so Allah, exalted is He, revealed this verse”.
Sa‘id ibn Jubayr said: “A Jew by the name of Malik ibn al-Sayf went to argue with the Prophet, Allah bless him and give him peace. The Prophet, Allah bless him and give him peace, said to him: ‘I plead with you by Him Who has revealed the Torah to Moses, do you not find in the Torah that Allah loathes the fat rabbi?’ Because this Jew was a fat rabbi, he got angry and said: ‘By Allah, Allah has never sent anything to any human being’. Horrified, his companions exclaimed: ‘Woe unto you, not even to Moses!’ He said: ‘By Allah, Allah has never sent anything to any human being’. And so Allah, exalted is He, revealed this verse”.
Asbab Al-Nuzul by Al-Wahidi , trans. Mokrane Guezzou