References
Hadiths of the Prophet
What the hadiths of the Prophet (peace be upon him) and the scholars say about the khumra.
Abdullah ibn Shaddad reported that Maymunah said: "The Prophet (peace be upon him) used to pray on the khumra."
Aishah said: "The Messenger of Allah (peace be upon him) said to me: 'Hand me the khumra from the mosque.' I said: 'I am menstruating.' He said: 'Your menstruation is not in your hand.'"
Abdullah ibn Shaddad said: Maymunah, the wife of the Prophet, said that when she was menstruating and not praying, she would lie down next to the place where the Messenger of Allah (peace be upon him) prayed, while he prayed on his khumra. "When he prostrated, part of his garment would touch me."
What the scholars say
In al-Nihayah, Ibn al-Athir wrote: "The khumra is a piece of matting or woven palm fronds, or similar plant material, large enough for a man to place his face on in prostration. It is only called a khumra at this size, and it was named khumra because its threads are covered (mastura) by its palm fronds."
Al-Azhari said the same in his Tahdhib, as did his companion Abu Ubayd al-Harawi and a group of scholars after them. Al-Nihayah adds: "It is only called a khumra at this size," and: "It was named khumra because its threads are covered by its palm fronds." Al-Khattabi said: "It is the prayer mat on which the worshipper prostrates."