Sayyiduna Abu Hurayrah (may Allah be pleased with him) says that in the time of the Prophet (peace and blessings be upon him) a man freed six slaves at the time of his death, and he had no other wealth. He (peace and blessings be upon him) divided them into parts; he (peace and blessings be upon him) freed two and kept four in slavery.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب العتق / 21399