سنن ابن ماجه

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).