Musnad Ahmad 482:166

And I heard the Messenger of Allah, peace and blessings of Allah be upon him, saying: "Two men from my nation: one of them rises at night and struggles with himself to perform ablution, while he has knots upon him. So he performs ablution; when he washes his hands, a knot is untied. When he washes his face, a knot is untied. When he wipes his head, a knot is untied. And when he washes his feet, a knot is untied. Then the Lord says to those behind the veil: 'Look at this servant of Mine; he struggles with himself. Whatever this servant of Mine asks of Me, it is for him.'"