When the Prophet ﷺ sat during the prayer, he would place his right hand on his knee and raise the finger next to his right thumb, supplicating with it, while his left hand rested on his left knee, spread out upon it.
حدثنا محمود بن غيلان ويحيى بن موسى وغير واحد قالوا حدثنا عبد الرزاق عن معمر عن عبيد الله بن عمر عن نافع
أن النبي ﷺ كان إذا جلس في الصلاة وضع يده اليمنى على ركبته ورفع إصبعه التي تلي الإبهام اليمنى يدعو بها ويده اليسرى على ركبته باسطها عليه
Its takhrij (source citations) Muslim also narrated it in Book of Mosques, Chapter on the manner of sitting in prayer and how the thighs are positioned, hadith no. 580, from the hadith of Abdul-Razzaq, through the same chain, and it is in Musannaf, hadith no. 3238.There are also narrations on this subject from Abdullah bin Zubayr, 3557 [Muslim, no. 579], Numayr Khuzai [Abu Dawud, no. 991], Abu Hurairah [to come, and Nasai, no. 1273], Abu Humayd [Abu Dawud, no. 734], and Waail bin Hajar [Abu Dawud, nos. 726, 957, Nasai, no. 888, Ibn Majah, no. 867].