Sahih Muslim Book 4, Hadith Number 1396.

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 angels invoke blessings on everyone among you so long as he is in a place of worship with these words: O Allah ! pardon him, O Allah, have mercy upon him, (and they continue to do so) as long as, he ablution (of the worshipper) is not broken, and one among you is in prayer and so long as he is detained for the prayer.