Musnad Abu Ya'la al-Mawsili 209:41

Sulayman ibn Abd al-Jabbar narrated to us, Abu Asim narrated to us, Abu Ka'b—meaning the owner of the silk—narrated to us, from Shahr ibn Hawshab, from Umm Salamah, who said: "Among the most frequent supplications of the Messenger of Allah (peace and blessings of Allah be upon him) when he was in my house was: 'O Turner of hearts, keep my heart firm upon Your religion.' She said: I said, 'O Messenger of Allah, why is this among your most frequent supplications?' He said: 'There is no heart except that it is between two fingers of the fingers of the Lord; if He wills, He makes it stand upright, and if He wills, He makes it deviate.'"