Musnad Ahmad 1153:591

Yunus narrated to us, he said: Hammad—meaning Ibn Zayd—narrated to us, from al-Mu‘alla ibn Ziyad, Hisham, and Yunus, from al-Hasan, that ‘Aishah said: There were supplications that the Messenger of Allah, peace and blessings of Allah be upon him, would frequently supplicate with: "O Turner of hearts, make my heart firm upon Your religion." She said: So I said: "O Messenger of Allah, you frequently supplicate with this supplication." He said: "Indeed, the heart of the son of Adam is between two fingers of the fingers of Allah, the Almighty and Majestic. If He wills, He makes it deviate, and if He wills, He makes it upright."