Musannaf Ibn Abi Shayba 21:77

Mu'adh narrated to us, Abu Ka'b the silk merchant informed us, 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 hearts, keep my heart firm upon Your religion.'" Then he said: "O Umm Salamah, there is no human being except that his heart is between two of the fingers of Allah; whom He wills, He keeps upright, and whom He wills, He causes to deviate."