Musnad Ahmad 32:167

Abu Mu'awiyah narrated to us, Al-A'mash narrated to us, from Abu Sufyan, from Anas, who said: The Prophet, peace and blessings of Allah be upon him, used to frequently say: "O Turner of the hearts, make my heart firm upon Your religion." He said: So we said, "O Messenger of Allah, we have believed in you and in what you have brought, so do you fear for us?" He said: So he said: "Yes, for the hearts are between two fingers of the fingers of Allah, the Almighty and Majestic, He turns them."