Musannaf Ibn Abi Shayba 21:560

Waki’ narrated to us, from ‘Isam ibn Qudama, from Malik ibn Numayr al-Khuza’i, from his father, who said: I saw the Messenger of Allah (peace and blessings of Allah be upon him) sitting in the prayer, placing his right hand upon his right thigh and his left hand upon his left thigh. He gestured with his index finger, placed his thumb upon his middle finger, and his left palm grasped his knee.