Abdullah bin Umar saw me fidgeting with pebbles during the prayer. When he finished, he forbade me from doing so and said, "Do as the Messenger of Allah ﷺ used to do."I asked, "How did the Messenger of Allah ﷺ do it?"He replied, "When he sat during the prayer, he would place his right palm on his right thigh, close all his fingers, and point with the finger next to the thumb, and he would place his left palm on his left thigh."
حدثنا القعنبي عن مالك عن مسلم بن أبي مريم
رآني عبد الله بن عمر وأنا أعبث بالحصى في الصلاة فلما انصرف نهاني وقال اصنع كما كان رسول الله ﷺ يصنع فقلت وكيف كان رسول الله ﷺ يصنع قال كان إذا جلس في الصلاة وضع كفه اليمنى على فخذه اليمنى وقبض أصابعه كلها وأشار بأصبعه التي تلي الإبهام ووضع كفه اليسرى على فخذه اليسرى
Muslim narrated it in the Book of Mosques, Chapter on the manner of sitting in prayer and how the hands are placed on the thighs, Hadith: 580, from the hadith of Malik through this same chain. It is also found in Muwatta (Yahya): 1/88, 89.