Musnad Abu Ya'la al-Mawsili 86:1

Abu Musa Muhammad ibn al-Muthanna narrated to us, Abu Dawud narrated to us, Abu 'Amir narrated to us, from al-Hasan, from Sa'd, the freed slave of Abu Bakr al-Siddiq, from the Messenger of Allah (may Allah bless him and grant him peace), who said to Abu Bakr—and Sa'd was a slave of his, and the Messenger of Allah (may Allah bless him and grant him peace) used to be pleased with his service—the Messenger of Allah (may Allah bless him and grant him peace) said: "Free Sa'd." Abu Bakr said: "O Messenger of Allah, we have no one here other than him." 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."