Musannaf Ibn Abi Shayba 3:155

Abu Bakr narrated to us, he said: Ghundar narrated to us, from Shu'bah, from Qays ibn Muslim, from Tariq ibn Shihab, he said: A man from the companions of the Prophet (peace be upon him) from the Ansar stood among us on the day of al-Qadisiyyah and said: "When one bows, let him place his hands upon his knees and steady himself until his tailbone rises."