Musnad Ahmad 1167:39

'Affan narrated to us, he said: Wuhayb narrated to us, he said: An-Nu'man ibn Rashid narrated to me, from the brother of Az-Zuhri, from a freed slave of Asma' bint Abi Bakr, from Asma', she said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "O assembly of women, whoever among you believes in Allah and the Last Day, let her not raise her head until the men raise their heads." She said: "And that was because their lower garments were short, for fear that their private parts would be exposed when they prostrated."