It is narrated from Sayyiduna Abdullah al-Muzani (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "A man may free as much of his slave as he wishes: if he wishes, a third, and if he wishes, a quarter."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 14071