Musannaf Ibn Abi Shayba 3:167

Abu Bakr narrated to us, he said: Waki’ narrated to us, he said: Ibn ‘Awn narrated to us, from Ibrahim, that the Prophet (peace and blessings of Allah be upon him) used to do it—meaning he would clasp his hands together in bowing (ruku’). Ibn ‘Affan said: So I mentioned it to Ibn Sirin, and he said: "Perhaps he did it once."