Musnad Abu Ya'la al-Mawsili 131:15

Abu Khaythama narrated to us, Jarir narrated to us, from al-Mukhtar, from Anas, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "People will continue to ask, 'What is this? What is this?' until they say, 'Allah created the creation, so who created Allah?'"