Musnad Ahmad 1156:207

Mu'adh ibn Mu'adh narrated to us, he said: Abu Ka'b, the owner of silk, narrated to us, he said: Shahr ibn Hawshab narrated to me, 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, make my heart firm upon Your religion.'" She said: "So I said to him: 'O Messenger of Allah, why do you frequently supplicate: O Turner of 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, the Almighty and Majestic. Whomever He wills, He makes upright, and whomever He wills, He causes to deviate." 'Abdullah said: I asked my father about Abu Ka'b, and he said: "Trustworthy, and his name is 'Abd Rabbih ibn 'Ubayd."