Sahih Muslim Book 1, Hadith Number 249.

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

Yazid b. al-Asamm said: I heard Abu Huraira saying that the Messenger of Allah (may peace be upon him) observed: people will certainly ask you about everything till they will propound: Allah created every thing, but who created Allah?