Sahih Muslim, Chapter: Faith (Kitab Al Iman)

The Messenger of Allah (may peace be upon him) said to me: they (the people) will constantly ask you, Abu Huraira, (about different things pertaining to religion) then they would say: Well, there is Allah, but after all who created Allah? He (Abu Huraira) narrated: Once we were in the mosque that some of the Bedouins came there and said: Well, there is Allah, but who created Allah? He (the narrator) said: I took hold of the pebbles in my fist and flung at them and remarked: Stand up, stand up (go away) my friend (the Holy Prophet) told the truth.