Sahih Muslim
The Book of Purification (Kitab Al-Taharah)
Book Number: 002 / Hadith Number: 475
Abu Huraira reported: Allah's Messenger (may peace be upon him) said: When a bondsman-a Muslim or a believer-washes his face (in course of ablution), every sin he contemplated with his eyes, will be washed away from his face along with water, or with the last drop of water; when he washes his hands, every sin they wrought will be effaced from his hands with the water, or with the last drop of water; and when he washes his feet, every sin towards which his feet have walked will be washed away with the water or with the last drop of water with the result that he comes out pure from all sins.