Musnad Ahmad 482:36

'Abd al-Wahhab al-Khaffaf narrated to us, from Sa'id, from Qatadah, who said: It was mentioned that Qays al-Judhami narrated from 'Uqbah ibn 'Amir al-Juhani, that the Messenger of Allah (peace and blessings of Allah be upon him) said: "Whoever frees a believing slave, it is his ransom from the Fire."