When the Prophet ﷺ sat for the tashahhud, he would place his left hand on his left knee and his right hand on his right knee, form the shape of fifty-three with his fingers, and make supplication.
حدثنا عفان حدثنا حماد بن سلمة أخبرنا أيوب عن نافع
أن النبي ﷺ كان إذا قعد يتشهد وضع يده اليسرى على ركبته اليسرى ووضع يده اليمنى على ركبته اليمنى وعقد ثلاثا وخمسين ودعا
Its isnad is sahih. Muslim related it, 1:162, through the route of Yunus bin Muhammad, from Hammad bin Salama, with this same isnad, but in his version the ending reads, "and he pointed with his index finger," instead of what is said here, "and he called upon Allah." See also 5421, 6000, and Nawawi's commentary on Muslim, 5:80-82.