From Sahih Muslim (Hadith 1311):
Ali b. 'Abual-Rahman al-Mu'awi reported:'Abdullah b. Umar saw me playing with pebbles during prayer. After finishing the prayer he forbade me (to do it) and said: Do as the Messenger of Allah (ﷺ) used to do. I said: How did Allah's Messenger (ﷺ) do? He said that he (the Messenger of Allah) sat at tashahhud, placed his right palm on the right thigh and closed all his fingers and pointed with the help of finger next to the thumb, and placed his left palm on his left thigh
Reference: Muslim 1311