Sahih Muslim Book 2, Hadith Number 437.

Chapter : How to perform ablution.

Humran, the freed slave of ‘Uthman said: I saw Uthman call for a vessel (of water) and poured water over his hands three times and then washed them. Then he put his right hand in the vessel and rinsed his mouth and cleaned his nose. Then he washed his face three times and his hands up to the elbow three times; then wiped his head, then washed his feet three times. Then he said that the Messenger of Allah (may peace be upon him) had said: He who performed ablution like this ablution of mine and offered two rak’ahs of prayer without allowing his thoughts to be distracted, all his previous sins would be expiated.