Sahih Muslim Book 19, Hadith Number 4453.

Chapter : Fighting of women side by side with men.

It has been narrated on the authority of Anas that, on the Day of Hunain. Umm Sulaim took out a dagger she had in her possession. Abiu Talha saw her and said: Messenger of Allah, this is Umm Sulaim. She is holding a dagger. The Messenger of Allah (may peace be upon him) asked (her): What for are you holding this dagger? She said: I took it up so that I may tear open the belly of a polytheist who comes near me. The Messenger of Allah (may peace be upon him) began to smile (at these words). She said: Messenger of Allah, kill all those people-other than us-whom thou hast declared to be free (on the day of the Conquest of Mecca). (They embraced Islam because) they were defeated at your hands (and as such their Islam is not dependable). The Messenger of Allah (may peace be upon him) said: Umm Sulaim. God is sufficient (against the mischief of the polytheists) and He will be kind to us (so you need not carry this dagger).