It is narrated from Sayyiduna Imran bin Husayn (may Allah be pleased with him) that a man freed six of his slaves at the time of his death, and the Prophet (peace and blessings be upon him) gave him an order, so he freed two and kept four in slavery.
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 15652