Sahih Muslim Book 2, Hadith Number 441.

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

‘Amr b Sa’id b al-As reported: I was with Uthman that he called for ablution water and said: I heard Allah’s Messenger (may peace be upon him) say: When the time for a prescribed prayer comes, if any Muslim perform ablution well and offers his prayer) with humility and bowing, it will be an expiation for his past sins, so long as he has not committed a major sin; and this applies to for all times.