Sahih Bukhari Volume 001, Book 004, Hadith Number 161.
Narated By Humran : (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 forearrlns 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 Apostle said 'If anyone Performs ablution like that of mine and offers a two-rak'at 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 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.