It is narrated from Hazrat Abu Hurairah (may Allah be pleased with him) that the Noble Prophet (peace and blessings be upon him) said: Whoever frees a Muslim slave, Allah will free every limb of the one who frees him from Hell in exchange for every limb of that slave, even so far as a hand for a hand, a foot for a foot, and the private part for the private part.
Hadith Referenceمسند احمد / مسند المكثرين من الصحابة / 9441