It is narrated from Sayyiduna Malik al-Qushayri (may Allah be pleased with him); he said that the Messenger of Allah (peace and blessings be upon him) said: "Whoever frees a Muslim slave, that will be his ransom from Hell: a bone of his for each bone of the one he freed."
Hadith Referenceالمعجم الكبير للطبراني / باب الميم / 16715