Musnad Ahmad 1153:2114

'Abd al-Samad and 'Affan narrated to us, they said: Hammad ibn Salamah narrated to us, he said: 'Ali ibn Zayd narrated to us, from Umm Muhammad, from 'Aishah, that the Messenger of Allah (peace and blessings of Allah be upon him) used to frequently say: "O Turner of hearts, make my heart firm upon Your religion and Your obedience." It was said to him: O Messenger of Allah—'Affan said: 'Aishah said to him—you frequently say: "O Turner of hearts, make my heart firm upon Your religion and Your obedience." He said: "And what makes me feel secure, while the hearts of the servants are between two of the fingers of the Most Merciful? If He wills to turn the heart of a servant, He turns it." 'Affan said: "Between two fingers from the fingers of Allah, the Mighty and Majestic."