Sahih Muslim Book 4, Hadith Number 1397.

Chapter : Permissibility of observing Nafl (supererogatory) prayer in congregation and that too on the mat or the covering cloth or any other thing which is free from filth and rubbish.

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.