Sahih Muslim Book 4, Hadith Number 1398.

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 the Messenger of Allah (may peace be upon him) as saying: Everyone among you is constantly in prayer so long as the prayer detains him (for this noble objective) and nothing prevents him to return to his family but the prayer.