It is narrated from Sayyiduna Ibn Umar (may Allah be pleased with them both) that the Messenger of Allah (peace and blessings be upon him) said: "Whoever frees his share of a slave, he will be obliged to free the slave completely according to the price set by just men."
Hadith Referenceمسند احمد / مسند المكثرين من الصحابة / 4451