كتاب الْمَسَاجِدِ وَمَوَاضِعِ الصَّلاَةِ
5  
The Book of Mosques and Places of Prayer
(48)
Chapter: 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
(48)
باب جَوَازِ الْجَمَاعَةِ فِي النَّافِلَةِ وَالصَّلاَةِ عَلَى حَصِيرٍ وَخُمْرَةٍ وَثَوْبٍ وَغَيْرِهَا مِنَ الطَّاهِرَاتِ
Sahih Muslim 659

Anas b. Malik reported that the Messenger of Allah (ﷺ) wits the best among people in character. On occasions, the time of prayer would come while he was in our house. He would then order to spread the mat lying under him. That was dusted and then water was sprinkled over it. The Messenger of Allah (ﷺ) then led the prayer and we stood behind him, and that mat was made of the leaves of date-palm.

وَحَدَّثَنَا شَيْبَانُ بْنُ فَرُّوخَ، وَأَبُو الرَّبِيعِ، كِلاَهُمَا عَنْ عَبْدِ الْوَارِثِ، قَالَ شَيْبَانُ حَدَّثَنَا عَبْدُ الْوَارِثِ، عَنْ أَبِي التَّيَّاحِ، عَنْ أَنَسِ بْنِ مَالِكٍ، قَالَ كَانَ رَسُولُ اللَّهِ صلى الله عليه وسلم أَحْسَنَ النَّاسِ خُلُقًا فَرُبَّمَا تَحْضُرُ الصَّلاَةُ وَهْوَ فِي بَيْتِنَا فَيَأْمُرُ بِالْبِسَاطِ الَّذِي تَحْتَهُ فَيُكْنَسُ ثُمَّ يُنْضَحُ ثُمَّ يَؤُمُّ رَسُولُ اللَّهِ صلى الله عليه وسلم وَنَقُومُ خَلْفَهُ فَيُصَلِّي بِنَا وَكَانَ بِسَاطُهُمْ مِنْ جَرِيدِ النَّخْلِ ‏.‏
Reference : Sahih Muslim 659
In-book reference : Book 5, Hadith 333
USC-MSA web (English) reference : Book 4, Hadith 1388
  (deprecated numbering scheme)