Musnad Abu Ya'la al-Mawsili 209:6

Al-Abbas ibn al-Walid narrated to us, Wahib narrated to us, from Khalid, from Abu Qilabah, from Zaynab bint Umm Salamah, from her mother, Umm Salamah: That the Prophet, may Allah bless him and grant him peace, used to pray on the khumrah (a small prayer mat made of palm leaves).