Musannaf Ibn Abi Shayba 27:698

Abu Mu'awiyah narrated to us, from Isma'il, from Qays, who said: Abu Bakr bought Bilal for five awqiyah, then he freed him. He said: Bilal said to him, "O Abu Bakr, if you only freed me to take me as a servant, then take me as a servant. But if you freed me for the sake of Allah, then leave me to work for Allah." He said: Abu Bakr wept, then said, "Rather, I freed you for the sake of Allah."