Sunan Ibn Majah, Chapter: The Chapters of Establishing the Prayer and the Sunnah Regarding Them

"A woman used to perform prayer behind the Prophet (saw), and she was one of the most beautiful of people. Some of the people used to go into the first row so that they would not see her, and some of them used to lag behind so that they would be in the last row, and when they bowed, they would do like this so that they could see her from beneath their armpits. Then Allah revealed: "And indeed, We know the first generations of you who had passed away, and indeed, We know the present generations of you (mankind), and also those who will some afterwards." [15:24] concerning her matter. Sahih