It was narrated that Anas bin Malik said: "One of my paternal uncles made some food for the Prophet (ﷺ) and said to the Prophet: 'I would like you to eat and perform prayer in my house.' So he went to him, and in his house there was one of these Fahl. He ordered that a corner be swept and water sprinkled in it, then he performed prayer and we prayed with him.'" (Sahih)Abu 'Abdullah bin Majah said: A Fahl is a mat that has become black (through use).
Source: sunnah.com
Other translations
Arabic
حَدَّثَنَا يَحْيَى بْنُ حَكِيمٍ، حَدَّثَنَا ابْنُ أَبِي عَدِيٍّ، عَنِ ابْنِ عَوْنٍ، عَنْ أَنَسِ بْنِ سِيرِينَ، عَنْ عَبْدِ الْحَمِيدِ بْنِ الْمُنْذِرِ بْنِ الْجَارُودِ، عَنْ أَنَسِ بْنِ مَالِكٍ، قَالَ صَنَعَ بَعْضُ عُمُومَتِي لِلنَّبِيِّ ـ صلى الله عليه وسلم ـ طَعَامًا فَقَالَ لِلنَّبِيِّ ـ صلى الله عليه وسلم ـ إِنِّي أُحِبُّ أَنْ تَأْكُلَ فِي بَيْتِي وَتُصَلِّيَ فِيهِ . قَالَ فَأَتَاهُ وَفِي الْبَيْتِ فَحْلٌ مِنْ هَذِهِ الْفُحُولِ فَأَمَرَ بِنَاحِيَةٍ مِنْهُ فَكُنِسَ وَرُشَّ فَصَلَّى وَصَلَّيْنَا مَعَهُ . قَالَ أَبُو عَبْدِ اللَّهِ بْنُ مَاجَهْ الْفَحْلُ هُوَ الْحَصِيرُ الَّذِي قَدِ اسْوَدَّ .
sunnah.com