Musnad Ahmad 1163:27

Hassan ibn Musa narrated to us, he said: Ibn Lahi'ah narrated to us, he said: Bukayr ibn al-Ashajj narrated to me, from Kurayb, the freed slave of Ibn Abbas, that he said: I heard Maymunah, the wife of the Prophet, may Allah bless him and grant him peace, saying: I freed a slave girl during the time of the Prophet, may Allah bless him and grant him peace, and I mentioned that to him. The Messenger of Allah, may Allah bless him and grant him peace, said to me: "If you had given her to your maternal uncles, it would have been a greater reward for you."