Sahih Muslim 135a

'Abd al-Warith ibn 'Abd al-Samad narrated to me: My father narrated to me, from my grandfather, from Ayyub, from Muhammad ibn Sirin, from Abu Hurayra, from the Prophet (peace and blessings be upon him), who said: People will keep asking you about knowledge until they say, Allah created us, so who created Allah? He said, while holding a man's hand: Allah and His Messenger have spoken the truth. Two people have asked me this already, and this is the third; or he said, One person has asked me, and this is the second.