Sahih Bukhari Volume 001, Book 006, Hadith Number 321.
Narated By Aiyub : Hafsa said, 'We used to forbid our young women to go out for the two 'Id prayers. A woman came and stayed at the palace of Bani Khalaf and she narrated about her sister whose husband took part in twelve holy battles along with the Prophet and her sister was with her husband in six (out of these twelve). She (the woman's sister) said, "We used to treat the wounded, look after the patients and once I asked the Prophet, 'Is there any harm for any of us to stay at home if she doesn't have a veil?' He said, 'She should cover herself with the veil of her companion and should participate in the good deeds and in the religious gathering of the Muslims.' When Um 'Atiya came I asked her whether she had heard it from the Prophet. She replied, "Yes. May my father be sacrificed for him (the Prophet)! (Whenever she mentioned the Prophet she used to say, 'May my father be sacrificed for him) I have heard the Prophet saying, 'The unmarried young virgins and the mature girl who stay often screened or the young unmarried virgins who often stay screened and the menstruating women should come out and participate in the good deeds as well as the religious gathering of the faithful believers but the menstruating women should keep away from the Musalla (praying place).' " Hafsa asked Um 'Atiya surprisingly, "Do you say the menstruating women?" She replied, "Doesn't a menstruating woman attend 'Arafat (Hajj) and such and such (other deeds)?"