Musnad Abu Ya'la al-Mawsili 155:216

Zuhayr narrated to us, Isma'il narrated to us, Ayyub narrated to us, from Muhammad ibn Sirin, who said: Abu Hurayrah said: "People will continue to ask about knowledge until they say: 'This is Allah who created us, so who created Allah?'" He said: Then he took a man by the hand and said: "Allah and His Messenger have spoken the truth. A man has already asked me about this, and this is the second [time], or two men [have asked], and this is the third."