When the Prophet ﷺ sat for the tashahhud, he placed his left hand on his left knee and his right hand on his right knee, formed the number fifty-three with his fingers, and pointed with his index finger.
أن النبي ﷺ إذا قعد في التشهد وضع يده اليسرى على ركبته اليسرى، ووضع يده اليمنى على ركبته اليمني، وعقد ثلاثة وخمسين، وأشار بالسبابة.
Muslim narrated it in "The Mosques" (580) through the route of Hammad bin Salama, from Ayub, from Nafi, from Ibn Umar, who mentioned it likewise.As for his statement, "he formed the number fifty-three," they explained this forming as folding the little finger, the ring finger, and the middle finger, while releasing the thumb toward the base of the index finger. In "Talkhis" (1/262) it says, "Its form is that the thumb is placed crosswise beneath the index finger."