Sahih Muslim Book 2, Hadith Number 444.

Chapter : The merit of Wudu and that of prayer after it.

Humran b. Abin reported: I used to fetch water for ‘Uthman for his purification. Never was there a day that he did not take a bath with a small quantity of water. And Uthman said: The Messenger of Allah (may peace be upon him) at the time of our returning from our prayer told us (certain things pertaining to purification). Mis’ar said: I find that it was afternoon prayer. He said: I do not know whether I should tell you a thing or keep quiet. We said: Messenger of Allah, tell us if it is good and if it is otherwise, Allah and His Apostle know better. Upon this he said: A Muslim who purifies (himself) and completes purification as enjoined upon him by Allah and then offers the prayers, that will be expiatious (of his sins he committed) between these (prayers).