Musnad Ahmad 1156:104

Hashim narrated to us, Abd al-Hamid narrated to us, he said: Shahr narrated to me, he said: I heard Umm Salamah narrating that the Messenger of Allah (peace and blessings of Allah be upon him) used to say frequently in his supplication: "O Allah, Turner of hearts, make my heart firm upon Your religion." She said: I said: "O Messenger of Allah, do hearts really turn?" He said: "Yes. There is no human being among the creation of Allah, the son of Adam, but his heart is between two fingers of the fingers of Allah. If Allah, the Mighty and Majestic, wills, He keeps it upright, and if He wills, He causes it to deviate. So we ask Allah, our Lord, that He does not cause our hearts to deviate after He has guided us, and we ask Him to grant us mercy from Himself; indeed, He is the Bestower." She said: I said: "O Messenger of Allah, will you not teach me a supplication that I may supplicate with for myself?" He said: "Yes. Say: O Allah, Lord of the Prophet Muhammad, forgive me my sin, remove the rage of my heart, and protect me from the misleading trials as long as You keep us alive."