Musnad Ahmad 31:176

Muhammad ibn Ja'far narrated to us, Shu'bah narrated to us, from Zayd Abu al-Hawwari, who said: I heard Abu al-Siddiq narrating from Abu Sa'id al-Khudri, who said: "We used to sell the mothers of children (umm al-walad) during the time of the Messenger of Allah, may Allah bless him and grant him peace."