Sahih Muslim
The Book of Faith (Kitab Al-Iman)
Book Number: 001 / Hadith Number: 250
Anas b. Malik transmitted it from the Messenger of Allah (may peace be upon him) that the Great and Glorious Allah said: Verily your people would constantly question about this and that till they would say: Well, it is Allah Who created the creation, but who created Allah?