Musnad Abu Ya'la al-Mawsili 209:108

Abu Khaythama narrated to us, Mu'adh ibn Mu'adh al-'Anbari narrated to us, Abu Ka'b the owner of the silk narrated to us, he said: Shahr ibn Hawshab narrated to me, he said: I asked Umm Salamah, I said: "O Mother of the Believers, what was the most frequent supplication of the Messenger of Allah (may Allah bless him and grant him peace) when he was with you?" She said: "His most frequent supplication was: 'O Turner of the hearts, make my heart firm upon Your religion.'" She said: "So I said to him: 'O Messenger of Allah, why is your supplication so frequently: O Turner of the hearts, make my heart firm upon Your religion?'" He said: "O Umm Salamah, there is no human being except that his heart is between two fingers of the fingers of Allah; whomsoever He wills, He makes upright, and whomsoever He wills, He causes to deviate."