Musnad Ahmad 38:19

Jarir narrated to us, from Mansur, from Rashid Abu Sa'd, from Sa'id ibn Abd al-Rahman ibn Abza, from his father, who said: "When the Messenger of Allah, peace and blessings of Allah be upon him, would sit in the prayer and supplicate, he would place his right hand upon his thigh, and then he would gesture with his finger when he supplicated."