Musannaf Ibn Abi Shayba 3:2275

Yahya ibn Abi Bukayr narrated to us, he said: Zuhayr ibn Muhammad narrated to us, from Abdullah ibn Muhammad ibn Aqil, from Said ibn al-Musayyib, from Abu Said, from the Prophet, peace and blessings of Allah be upon him, who said: "O assembly of women, when the men prostrate, lower your gazes, so that you do not see the private parts of the men due to the tightness of the lower garments."