Musannaf Ibn Abi Shayba 3:2274

Husayn ibn Ali narrated to us, from Za'idah, from Abdullah ibn Muhammad ibn Aqil, from Jabir, 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."