Sahih Bukhari Volume 003, Book 031, Hadith Number 155.

Narated By Humran : I saw 'Uthman performing ablution; he washed his hands thrice, rinsed his mouth and then washed his nose, by putting water in it and then blowing it out, and washed his face thrice, and then washed his right forearm up to the elbow thrice, and then the left-forearm up to the elbow thrice, then smeared his head with water, washed his right foot thrice, and then his left foot thrice and said, "I saw Allah's Apostle performing ablution similar to my present ablution, and then he said, 'Whoever performs ablution like my present ablution and then offers two Rakat in which he does not think of worldly things, all his previous sins will be forgiven."