Sahih Muslim
The Book of Prayers (Kitab Al-Salat)
Book Number: 004 / Hadith Number: 1397
Abu Huraira reported Allah's Messenger (may peace be upon him) as saying: The servant is constantly in prayer so long as he is in a place of worship waiting for the prayer (to be observed in congregation), and the angels invoke (blessings upon him in these words): O Allah! pardon him. O Allah! show mercy to him, (and they continue to do so) till he returns (from the mosque having completed the prayer) or his ablution breaks. I said: How is the ablution broken? He said: By breaking of the wind noiselessly or with noise.