Sahih Bukhari, Chapter: Ablutions (Wudu')

(the slave of Uthman) I saw Uthman bin Affan asking for a tumbler of water (and when it was brought) he poured water over his hands and washed them thrice and then put his right hand in the water container and rinsed his mouth, washed his nose by putting water in it and then blowing it out. then he washed his face and forearms up to the elbows thrice, passed his wet hands over his head and washed his feet up to the ankles thrice. Then he said, "Allah's Messenger (saw) said 'If anyone Performs ablution like that of mine and offers a two rakat prayer during which he does not think of anything else (not related to the present prayer) then his past sins will be forgiven.'" After performing the ablution Uthman said, "I am going to tell you a Hadith which I would not have told you, had I not been compelled by a certain Holy Verse (the sub narrator Urwa said: This verse is: "Verily, those who conceal the clear signs and the guidance which we have sent down...)" (2:159). I heard the Prophet (saw) saying, 'If a man performs ablution perfectly and then offers the compulsory congregational prayer, Allah will forgive his sins committed between that (prayer) and the (next) prayer till he offers it.