Sahih Muslim
The Book of Purification (Kitab Al-Taharah)
Book Number: 002 / Hadith Number: 447
Humran, the freed slave of 'Uthman b. 'Affan, reported on the authority of 'Uthman b. 'Affan that he heard Allah's Messenger (may peace be upon him) say: He who performed ablution for prayer and performed it properly and then went (to observe) obligatory prayer and offered it along with people or with the congregation or in the mosque, Allah would pardon his sins.