Sahih Muslim Book 1, Hadith Number 250.

Chapter : Pertaining to evil suggestion or prompting in faith and what should be said when it occurs to the mind of a man.

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?