Musannaf Ibn Abi Shayba 21:559

Abdullah ibn Idris narrated to us, from Asim ibn Kulayb, from his father, from Wa'il ibn Hujr, who said: I saw the Prophet, may Allah bless him and grant him peace, place the edge of his right elbow upon his right thigh. And he formed a circle with his thumb and middle finger, and he raised the one next to the thumb, supplicating with it.