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
حَدَّثَنَا يَحْيَى بْنُ يَحْيَى التَّمِيمِيُّ، أَخْبَرَنَا خَالِدُ بْنُ عَبْدِ اللَّهِ، ح وَحَدَّثَنَا أَبُو بَكْرِ بْنُ أَبِي شَيْبَةَ، قَالَ حَدَّثَنَا عَبَّادُ بْنُ الْعَوَّامِ، كِلاَهُمَا عَنِ الشَّيْبَانِيِّ، عَنْ عَبْدِ اللَّهِ بْنِ شَدَّادٍ، قَالَ حَدَّثَتْنِي مَيْمُونَةُ، زَوْجُ النَّبِيِّ صلى الله عليه وسلم قَالَتْ كَانَ رَسُولُ اللَّهِ صلى الله عليه وسلم يُصَلِّي وَأَنَا حِذَاءَهُ وَرُبَّمَا أَصَابَنِي ثَوْبُهُ إِذَا سَجَدَ وَكَانَ يُصَلِّي عَلَى خُمْرَةٍ .
Maimuna, the wife of the Messenger of Allah (ﷺ), reported: The Messenger of Allah (ﷺ) said prayer while I was by his side, and at times when he prostrated his cloth touched me, and he prayed on a small mat.