Prayer (Kitab Al-Salat): Details of Commencing Prayer
Dawud :: Book 3 : Hadith 850
Narrated Asma' daughter of AbuBakr:
I heard the Apostle of Allah (peace_be_upon_him) say: One of you who believes in Allah and in the Last Day should not raise her head until the men raise their heads (after prostration) lest they should see the private parts of men.