Musnad Ahmad 26:495

Ibn Numayr narrated to us, al-A‘mash narrated to us, from Ibrahim, from al-Aswad, who said: I and ‘Alqamah entered upon ‘Abdullah ibn Mas‘ud, and he said: "When one of you bows, let him spread his forearms upon his thighs, for it is as if I am looking at the interlacing of the fingers of the Messenger of Allah, may Allah bless him and grant him peace, in prayer."