5. The Book of Mosques and Places of Prayer - كتاب الْمَسَاجِدِ وَمَوَاضِعِ الصَّلاَةِ
48) 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
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.