Musannaf Ibn Abi Shayba 23:98

Mu'adh ibn Mu'adh narrated to us, he said: Abu Ka'b, the owner of the silk, informed us, he said: Shahr ibn Hawshab narrated to us, he said: I said to Umm Salamah, "O Mother of the Believers, what was the most frequent supplication of the Messenger of Allah, peace and blessings of Allah be upon him, when he was with you?" She said: His most frequent supplication was, "O Turner of the hearts, keep my heart firm upon Your religion." I said, "O Messenger of Allah, why is your supplication so frequently, 'O Turner of the hearts, keep my heart firm upon Your religion'?" He said, "O Umm Salamah, there is no human being except that his heart is between two of the fingers of Allah; whomsoever He wills, He keeps upright, and whomsoever He wills, He causes to deviate."