Musnad Ahmad 31:134

Yahya ibn Adam narrated to us, Sharik narrated to us, from Abdullah ibn Muhammad ibn Aqil, from Sa’id ibn al-Musayyib, from Abu Sa’id al-Khudri, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "The best of the men's rows is the first row, and the worst of them is the last row. And the best of the women's rows is the last row, and the worst of them is the first row." And he said: "O assembly of women, do not raise your heads when you prostrate, so that you do not see the private parts of the men due to the tightness of the lower garments."