Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) says that the Messenger of Allah (peace and blessings be upon him) said: "Any man who has a share in a slave and frees his share, it is his duty to free the remainder as well at a fair value."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب العتق / 21346