Sahih Muslim Book 4, Hadith Number 1399.

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) said: Anyone amongst you who sat in a place of worship waiting for the prayer is in prayer and his ablution is not broken, the angels invoke blessing upon him (in these words): O Allah !pardon him. O Allah ! have mercy upon him.