Musnad Abu Ya'la al-Mawsili 150:165

Ibrahim ibn al-Hajjaj narrated to us, Hammad ibn Salamah narrated to us, from Ali ibn Zayd, from Umm Muhammad, from Aisha, that the Prophet, peace and blessings of Allah be upon him, used to frequently say: "O Turner of the hearts, keep my heart firm upon Your religion and Your obedience." I said: "O Messenger of Allah, you frequently supplicate with this, do you fear?" He said: "And what makes me feel secure, while the hearts of the servants are between two fingers of the fingers of Allah? If He wills to turn the heart of a servant, He turns it."