Musannaf Ibn Abi Shayba 27:301

Abu Mu’awiyah narrated to us, from Hisham ibn ‘Urwah, from his father, who said: Abu Bakr freed seven [people] who were being tortured for the sake of Allah: ‘Amir ibn Fuhayrah, Bilal, Zinnirah, Umm ‘Ubays, al-Nahdiyyah and her sister, and Harithah bint Mu’ammil.