It is narrated from Sayyiduna Abu Hurayrah (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) said: "Whoever frees a believing slave, Allah will free every limb of his from the Fire in return for a limb of that slave, even his private parts in return for the private parts (of the freed slave)."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الوصايا / 12597