Musnad Ahmad 26:41

Abu Mu'awiyah narrated to us, Al-A'mash narrated to us, from Ibrahim, from Al-Aswad and Alqamah, from Abdullah, who said: "When one of you bows, let him spread his forearms upon his thighs, and let him hunch his back, then let him clasp his hands together. 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." He said: "Then he clasped his hands together and showed them [to the people]."