Musnad Abu Ya'la al-Mawsili 150:454

Abu Musa also narrated to us, [he said] Abu Usamah narrated to us, from Hisham ibn Urwah, from his father, from Aisha, she said: The mattress of the Prophet, may Allah bless him and grant him peace, upon which we used to sleep, was made of tanned leather, stuffed with palm fibers.