Musannaf Ibn Abi Shayba 23:97

Abu Mu'awiyah narrated to us, from al-A'mash, from Abu Sufyan, from Anas, that the Prophet (peace be upon him) used to frequently say: "O Turner of the hearts, make my heart firm upon Your religion." I said: "O Messenger of Allah, we have believed in you and in what you have brought, so do you fear for us?" He said: "Indeed, the hearts are between two fingers of the fingers of Allah; He turns them."