The Messenger of Allah ﷺ was asked during the expedition of Tabuk about the sutrah of the one praying, and he said, "Like the rear of a saddle."
أن رسول الله ﷺ سئل في غزوة تبوك عن سترة المصلي فقال: كمؤخرة الرحل.
Muslim reported it in the Book of Prayer (500) through Abu Aswad Muhammad bin Abdul-Rahman, from Urwa, from Ayesha, who narrated the same.