Musnad Abu Ya'la al-Mawsili 128:3

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