Narrated Ibn ‘Abbas (Radi-Allahu ‘anhu):
I brought water to Uthman bin ‘Affan RA to perform the ablution while he was sitting on his seat. He performed the ablution in a perfect way and said, “I saw the Prophet (Sallallahu ‘Alaihi Wa Sallam) performing the ablution in this place and he performed it in a perfect way and said, “Whoever performs the ablution as I have done this time and then proceeds to the mosque and offers a two-Rak’at prayer and then sits there (waiting for the compulsory congregational prayers), then all his past sins will be forgiven.” The Prophet (Sallallahu ‘Alaihi Wa Sallam) further added, “Do not be conceited (thinking that your sins will be forgiven because of your prayer).”

Bukhari Vol. 8 : No. 441