بُلُوغُ الْمَرَامِ
Bulugh al-Maram 229
The Book of Prayer
وَعَنْ عَائِشَةَ - رَضِيَ اَللَّهُ عَنْهَا- قَالَتْ : { سُئِلَ رَسُولُ اَللَّهِ - صلى الله عليه وسلم -- فِي غَزْوَةِ تَبُوكَ - عَنْ سُتْرَةِ اَلْمُصَلِّي . فَقَالَ : "مِثْلُ مُؤْخِرَةِ اَلرَّحْلِ } أَخْرَجَهُ مُسْلِمٌ . [1]
Narrated 'Aishah (RA): Allah's Messenger (ﷺ) was asked in the expedition of Tabuk about the Sutra of the person who is offering the prayer; he said , "It is (something) like the back of a saddle." [Reported by Muslim].
Sunnah.com reference: Book 2, Hadith 96