Musnad Ahmad 30:2316

Makki ibn Ibrahim narrated to us, Abdullah—meaning Ibn Sa’id ibn Abi Hind—narrated to us, from Isma’il ibn Abi Hakim, the freed slave of the family of Al-Zubayr, from Sa’id ibn Marjanah, that he heard Abu Hurayrah say: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Whoever frees a believing slave, Allah will free for every limb of his, a limb of his from the Fire, to the extent that He will free the hand for the hand, the foot for the foot, and the private part for the private part." ‘Ali ibn Husayn said: "Did you hear this from Abu Hurayrah?" Sa’id said: "Yes." Then ‘Ali ibn Husayn said to a servant of his, who was the most intelligent of his servants: "Call Mutarrif for me." He said: When he stood before him, he said: "Go, for you are free for the sake of Allah, the Almighty and Majestic."