Musannaf Ibn Abi Shayba 3:204

Abu Bakr narrated to us, he said: Ibn ‘Ulayyah narrated to us, from Ibn ‘Awn, from Ibn Sirin, who said: It is sufficient for the bowing (ruku‘) when one places his hands firmly upon his knees, and for the prostration (sujud) when one places his forehead firmly upon the ground.