Sahih Muslim 658
Sahih Muslim · Book 5: 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 Holy Prophet) ate out of that and then said:
حَدَّثَنَا يَحْيَى بْنُ يَحْيَى ، قَالَ قَرَأْتُ عَلَى مَالِكٍ عَنْ إِسْحَاقَ بْنِ عَبْدِ اللَّهِ بْنِ أَبِي طَلْحَةَ ، عَنْ أَنَسِ بْنِ مَالِكٍ ، أَنَّ جَدَّتَهُ، مُلَيْكَةَ دَعَتْ رَسُولَ اللَّهِ صلى الله عليه وسلم لِطَعَامٍ صَنَعَتْهُ فَأَكَلَ مِنْهُ ثُمَّ قَالَ " قُومُوا فَأُصَلِّيَ لَكُمْ " . قَالَ أَنَسُ بْنُ مَالِكٍ فَقُمْتُ إِلَى حَصِيرٍ لَنَا قَدِ اسْوَدَّ مِنْ طُولِ مَا لُبِسَ فَنَضَحْتُهُ بِمَاءٍ فَقَامَ عَلَيْهِ رَسُولُ اللَّهِ صلى الله عليه وسلم وَصَفَفْتُ أَنَا وَالْيَتِيمُ وَرَاءَهُ وَالْعَجُوزُ مِنْ وَرَائِنَا فَصَلَّى لَنَا رَسُولُ اللَّهِ صلى الله عليه وسلم رَكْعَتَيْنِ ثُمَّ انْصَرَفَ .
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 Holy 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.