سنن ابن ماجه

Humran the freed slave of 'Uthman bin 'Affan said: "I saw 'Uthman bin 'Affan sitting in Maqa'id. He called for water and he performed ablution, the he said: 'I saw the Messenger of Allah (ﷺ) sitting in this place where I am sitting, performing ablution as I have done. Then he said: "Whoever performs ablution as I have done, his previous sins will be forgiven." And the Messenger of Allah (ﷺ) said: "And do not be conceited (due to this great virtue)." (Sahih) Another chain with similar wording.