Musnad Abu Ya'la al-Mawsili 131:23

Abu Bakr narrated to us, Husayn ibn Ali narrated to us, from Za'idah, from al-Mukhtar ibn Fulful, from Anas, from the Prophet (peace and blessings of Allah be upon him), who said: "Indeed, Allah has said: 'Your nation will continue to ask questions, saying: "What is this? What is this?" until they say: "This is Allah, who created everything, so who created Allah?"'"