Sunan Ibn Majah, Chapter: The Chapters of Purification and its Sunnah

"The Messenger of Allah (saw) said: 'When a person performs ablution and washes his hands, his sins exit through his hands. When he washes his face, his sins will exit through his face. When he washes his forearms and wipes his head, his sins will exit through his forearms and head. When he washes his feet, his sins exit through his feet.'" (Hasan)