The Messenger of Allah ﷺ was asked during the Expedition of Tabuk about the sutrah of one performing prayer, and he said, "Like the back of a saddle."
أخبرنا العباس بن محمد الدوري قال حدثنا عبد الله بن يزيد قال حدثنا حيوة بن شريح عن أبي الأسود عن عروة
سئل رسول الله ﷺ في غزوة تبوك عن سترة المصلي فقال مثل مؤخرة الرحل
Muslim narrated it in the chapter on prayer, in the section on the barrier of the one praying and the encouragement to pray toward a barrier, etc., hadith 244/500, from the hadith of Abdullah bin Yazid Muqri, through him. It is also found in Kubra, hadith 821.