Musnad Ahmad 27:1540

Muhammad ibn Abdullah Abu Ahmad al-Zubayri narrated to us, Kathir ibn Zayd narrated to us, from Nafi’ who said: When Abdullah ibn Umar would sit in prayer, he would place his hands upon his knees, point with his finger, and follow it with his gaze. Then he said: The Messenger of Allah (ﷺ) said: "It is more severe upon Satan than iron," meaning the index finger.