Musnad Ahmad 30:1906

'Affan narrated to us, Abu 'Awanah narrated to us, from 'Umar ibn Abi Salamah, from his father, from Abu Hurayrah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "They will continue to ask until it is said: 'This is Allah who created us, so who created Allah, the Mighty and Majestic?'" Abu Hurayrah said: "By Allah, I was sitting one day when a man from the people of Iraq said to me: 'This is Allah who created us, so who created Allah, the Mighty and Majestic?'" Abu Hurayrah said: "So I placed my fingers in my ears, then I shouted and said: 'Allah and His Messenger have spoken the truth. Allah is the One, the Eternal Refuge. He neither begets nor is He begotten, and there is none co-equal or comparable unto Him.'"