Musnad Ahmad 3:17

Muhammad ibn Ja‘far narrated to us, Sa‘id narrated to us, from Qatadah, from Muslim ibn Yasar, from Humran ibn Aban, from ‘Uthman ibn ‘Affan: That he called for water, then performed ablution. He rinsed his mouth and snuffed water into his nose, then washed his face three times, and his forearms three times each, and wiped his head, and the tops of his feet. Then he laughed and said to his companions: "Will you not ask me what made me laugh?" They said: "What made you laugh, O Commander of the Faithful?" He said: "I saw the Messenger of Allah, may Allah bless him and grant him peace, call for water near this spot, then he performed ablution as I performed ablution, then he laughed and said: 'Will you not ask me what made me laugh?'" They said: "What made you laugh, O Messenger of Allah?" He said: "Indeed, when the servant calls for ablution and washes his face, Allah removes from him every sin he committed with his face. When he washes his forearms, it is likewise. If he wipes his head, it is likewise. And when he cleanses his feet, it is likewise."