Musnad Ahmad 3:61

Hassan ibn Musa narrated to us, Shaiban narrated to us, from Yahya, from Muhammad ibn Ibrahim ibn al-Harith al-Taymi, who said: Mu'adh ibn Abd al-Rahman informed me that Humran ibn Aban informed him, saying: I came to Uthman ibn Affan while he was sitting in the seating area. He performed ablution and performed the ablution well. Then he said: I saw the Messenger of Allah, peace and blessings of Allah be upon him, while he was in this seat, perform ablution and perform the ablution well. Then he said, and he said: "Whoever performs ablution like this ablution of mine, then comes to the mosque and performs two rak'ahs in it, his past sins will be forgiven for him." And he said: The Messenger of Allah, peace and blessings of Allah be upon him, said: "And do not be deceived."