Musnad Abu Ya'la al-Mawsili 150:57

Muhammad ibn Abdullah narrated to us, Abdullah ibn Yazid narrated to us, Haywah narrated to us, from Abu al-Aswad, from Urwah, from Aisha: That the Messenger of Allah, peace and blessings of Allah be upon him, was asked during the Battle of Tabuk about the sutrah (barrier) of the one praying, and he said: "Like the back part of a saddle."