Sahih Muslim
The Book of Purification (Kitab Al-Taharah)
Book Number: 002 / Hadith Number: 446
Humran, the freed slave of Uthman reported: One day Uthman b. Affan performed the ablution well, and then said: I saw Allah's Messenger (may peace be upon him) perform ablution, the best ablution, and then observed: He who performed ablution like this and then went towards the mosque and nothing (but the love of) prayer urged him (to do so), all his previous (minor) sins would be expiated.