The Prophet ﷺ, when he sat during prayer, would place his hands on his knees and raise the finger next to his thumb on the right hand, supplicating with it, while his left hand rested spread out on his left knee.In another narration, when he sat for the tashahhud, he placed his left hand on his left knee and his right hand on his right knee, formed the shape of fifty-three with his fingers, and pointed with his index finger.
أن النبي ﷺ لو كان إذا جلس في الصلاة وضع يديه على ركبتيه، ورفع إصبعه اليمنى التي تلى الإبهام، فدعا بها، ويده اليسرى على ركبته اليسرى باسطها عليها. وفي رواية: كان إذا قعد في التشهد وضع يده اليسرى على ركبته اليسرى، ووضع يده اليمنى على ركبته اليمنى، وعقد ثلاثة وخمسين، وأشار بالسبابة.
Muslim reported it in "the Mosques" chapter (580). The first narration is via Abdul-Razzaq, who said, Mamar informed us, from Ubaydullah bin Umar, from Nafi, from Ibn Umar, and he related it. The narration is also transmitted via Hammad bin Salama, from Ayub, from Nafi, from Ibn Umar, and he related it.