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 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 said: "And do not be conceited (due to this great virtue)." (Sahih)Another chain with similar wording. (IT)
English reference (IT): Vol. 1, Book 1, Hadith 285 (IT)
Arabic reference (IT): Book 1, Hadith 299 (IT)