The Messenger of Allah ﷺ said, "When one of you places before him something like the back of a saddle, let him pray, and he need not be concerned about whoever passes beyond it."
قال رسول الله ﷺ: إذا وضع أحدكم بين يديه مثل مؤخرة الرحل فليصل. ولا يبالي من مر وراء ذلك.
Muslim narrated it in the Book of Prayer (499) through the route of Abu Ahwas, from Simak, from Musa bin Talha, from his father, and he mentioned similar to it. The second narration is through the route of Umar bin Ubayd Tanafisi, from Simak, with the same wording. Ibn Khuzayma narrated it through this same route (805), and from him Ibn Hibban (2380), in their two Sahihs.Ataa said, "The rear part of the saddle is a forearm's length or more." Abu Dawud recorded it with a chain (686) through the route of Abdul-Razzaq, from Ibn Jurayj, from Ataa, who mentioned similar to it.I say, the rear part of the saddle is the piece of wood that the rider leans against, as Nawawi mentioned.