Musnad Ahmad 15:2

Sulayman ibn Dawud narrated to us, Abu 'Amir narrated to us, from al-Hasan, from Sa'd, the freed slave of Abu Bakr, and he used to serve the Prophet, may Allah bless him and grant him peace, and the Prophet, may Allah bless him and grant him peace, used to be pleased with his service. He said: "O Abu Bakr, free Sa'd." He said: "O Messenger of Allah, we have no servants other than him." He said: The Messenger of Allah, may Allah bless him and grant him peace, said: "Free Sa'd; men will come to you, men will come to you." Abu Dawud said: Meaning captives.