Musnad Ahmad 1167:40

Surayj ibn al-Nu‘man narrated to us, he said: Sufyan ibn ‘Uyaynah narrated to us, from al-Zuhri, from ‘Urwah, from Asma’ bint Abi Bakr, 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 Imam raises his head," [due to] the tightness of the men's garments.