Sunan at-Tirmithiy كتاب النذور والأيمان باب ما جاء كيف كان يمين النبى
The Messenger of God, peace and blessings of God be upon him, often swore by this oath, "No, by the One who turns the hearts (1)." Footnotes: (1) i.e. The One who causes the hearts to change from disbelief to belief and vice versa. Click on ḥadīth to read more