Musnad Ahmad 1153:2249

Abu Ahmad Muhammad ibn Abdullah ibn al-Zubayr narrated to us, he said: Mis'ar narrated to us, from Ubayd ibn Hasan, from Ibn Ma'qil, from Aisha, that she had an obligation to free a slave from the descendants of Ishmael. Then captives came from Yemen, from the tribe of Khawlan, and she wanted to free one of them, but the Prophet, peace and blessings of Allah be upon him, forbade me. Then captives came from Mudar, from the tribe of Banu al-Anbar, and the Prophet, peace and blessings of Allah be upon him, ordered her to free one of them.