Sahih Muslim Book 002, Hadith Number 0436.
Chapter : How to perform ablution.
Humran, the freed slave of 'Uthman, said: Uthman b. 'Affan called for ablution water and this is how he performed the ablution. He washed his hands thrice. He then rinsed his mouth and cleaned his nose with water (three times). He then washed his face three times, then washed his right arm up to the elbow three times, then washed his left arm like that, then wiped his head; then washed his right foot up to the ankle three times, then washed his left foot like that, and then said: I saw the Messenger of Allah (may peace be upon him) perform ablution like this ablution of mine. Then the Messenger of Allah (may peace be upon him) said: He who performs ablution like this ablution of mine and then stood up (for prayer) and offered two rak'ahs of prayer without allowing his thoughts to be distracted, all his previous sins are expiated. Ibn Shihab said: Our scholars remarked: This is the most complete of the ablutions performed for prayer.