Musnad Ahmad 547:2

And I heard the Prophet, peace and blessings of Allah be upon him, saying: "Two men from my nation, one of them rises at night, struggling 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 Allah says to those behind the veil: 'Look at this servant of Mine; he struggles with himself and asks of Me. Whatever My servant asks of Me, it shall be his.'"