Sunan Abu Dawood, Chapter: Prayer (Kitab Al-Salat)

When the Prophet (peace_be_upon_him) proceeded for the Battle of Badr, I said to him: Apostle of Allah allow me to accompany you in the battle. I shall act as a nurse for patients. It is possible that Allah might bestow martyrdom upon me. He said: Stay at your home. Allah, the Almighty , will bestow martyrdom upon you. The narrator said: Hence she was called martyr. She read the Qur'an. She sought permission from the Prophet (peace_be_upon_him) to have a mu'adhdhin in her house. He, therefore, permitted her (to do so). She announced that her slave and slave-girl would be free after her death. One night they went to her and strangled her with a sheet of cloth until she died, and they ran away. Next day Umar announced among the people, "Anyone who has knowledge about them, or has seen them, should bring them (to him)." Umar (after their arrest) ordered (to crucify them) and they were crucified. This was the first crucifixion at Medina.