Sayyidah Asma bint Abi Bakr (may Allah be pleased with her) said: The Messenger of Allah (peace and blessings be upon him) said: "O believing women! Let no woman raise her head before the imam raises his head." This was because of the scantiness of the men's garments.
Hadith Referenceالمعجم الكبير للطبراني / مسند النساء / 20633