Musnad Ahmad 1163:22

Ya'la narrated to us, Muhammad—meaning Ibn Ishaq—narrated to us, from Bukayr ibn Abdullah ibn al-Ashajj, from Sulayman ibn Yasar, from Maymunah, the wife of the Prophet, may Allah bless him and grant him peace, who said: I emancipated a slave girl of mine. The Prophet, may Allah bless him and grant him peace, entered upon me, and I informed him of her emancipation. He said: "May Allah reward you. Had you given her to your maternal uncles, it would have been a greater reward for you."