Sunan An-Nasai, Chapter: The Book of Menstruation and Istihadah

"Umm Atiyah would never mention the Messenger of Allah (saw) without saying: 'May my father be ransomed for him.' I said: 'Did you hear the Messenger of Allah (saw) say such and such?' And she said: 'Yes, may my father be ransomed for him.' He said: 'Let the mature girls, virgins staying in seclusion, and menstruating women go out and witness the good occasions and the supplications of the Muslims, but let menstruating women keep away from the prayer place.'" (Sahih)