Musnad Abu Ya'la al-Mawsili 109:230

Abu Khaythama narrated to us, Rawh ibn ‘Ubadah narrated to us, Ibn Jurayj narrated to us, he said: Abu al-Zubayr informed me that he heard Jabir saying: We used to sell our concubines, the mothers of children (umm al-walad), while the Prophet, may Allah bless him and grant him peace, was alive among us, and he did not see any harm in that.