صحيح البخاري

Narrated 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 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 (ﷺ) 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.' "