Sahih Muslim 658
The Book of Mosques and Places of Prayer
It is permissible to offer voluntary prayers in congregation, and to pray on hasir (palm-fiber mats), khumrah (small mats), cloth, and other pure things
Anas b. Malik reported that his grandmother, Mulaika, invited the Messenger of Allah (ﷺ) to a dinner which she had prepared. He (the Prophet) ate out of that and then said: Stand up so that I should observe prayer (in order to bless) you Anas b. Malik said: I stood up on a mat (belonging to us) which had turned dark on account of its long use. I sprinkled water over it (in order to soften it), and the Messenger of Allah (ﷺ) stood upon it, and I and an orphan formed a row behind him (the Prophet) and the old woman was behind us, and the Messenger of Allah (ﷺ) led us in two rak'ahs of prayer and then went back.