Musnad Abu Ya'la al-Mawsili 107:27

Abu Khaythama narrated to us, Abd al-Samad ibn Abd al-Warith narrated to us, Hisham narrated to us, from Qatada, from Qays al-Judhami, from Uqba ibn Amir, who said: 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."