Musannaf Ibn Abi Shayba 3:195

Abu Bakr narrated to us, he said: Waki’ narrated to us, from Sufyan, from Ziyad al-Musaffar, from al-Hasan, from Ibn Mas’ud, who said: Three glorifications (tasbihat) in the bowing (ruku’) and the prostration (sujud) is the middle (standard/moderate) amount.