(Had we sent down unto thee (Muhammad) (actual) writing upon parchment…) [6:7].
Said al-Kalbi: “The idolaters of Mecca said: ‘O Muhammad, by Allah we will not believe until you bring us a book from Allah along with four angels with it, bearing witness that it is from Allah and that you are His messenger’, and so this verse was revealed”.
Asbab Al-Nuzul by Al-Wahidi , trans. Mokrane Guezzou