Musannaf Ibn Abi Shayba 21:79

Yazid narrated to us, Hammam ibn Yahya informed us, from Ali ibn Zayd, from Umm Muhammad, from Aisha, who said: The Messenger of Allah (peace and blessings of Allah be upon him) used to say: "O Turner of the hearts, make my heart firm upon Your religion." I said: "O Messenger of Allah, you supplicate with this supplication?" He said: "O Aisha, do you not know that the hearts—or he said, the heart of the son of Adam—are between two of the fingers of Allah? If He wills to turn it toward guidance, He turns it, and if He wills to turn it toward misguidance, He turns it."