Abu Hurayrah (may Allah be pleased with him) said: I heard the Messenger of Allah (peace and blessings be upon him) say: "Whoever frees a believing slave, Allah Almighty will free all of his limbs from Hell in return for it, even the private part for the private part."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب العتق / 21307