Musannaf Ibn Abi Shayba 3:202

Abu Bakr narrated to us, he said: Waki’ narrated to us, from al-A’mash, from the one who heard Muhammad ibn ‘Ali saying: It suffices him for the bowing (ruku’) when he places his hands upon his knees, and for the prostration (sujud) when he places his forehead upon the ground.