Sahih Muslim Book 19, Hadith Number 4428.

Chapter : The persecution of the Holy Prophet (may peace be upon him) at the hands of the infidels and hypocrites.

It has been narrated on the authority of Aswad b. Qais who heard Jundub saying that Gabriel delayed his visit to the Messenger of Allah (may peace be upon him) The polytheists began to say that Muhammad has been forsaken. At this Allah, the Glorious and Exalted, revealed: “Wa’dd hd wa’l-laili iza saja, ma wadda’ka Rabbuka wa’ ma qala” [By the glorious morning light, and by the night when it is still: thy Lord has not forsaken thee, nor is He displeased].