Sayyiduna Ibn Umar (may Allah be pleased with them both) narrates from the Prophet (peace and blessings be upon him) that the Messenger of Allah (peace and blessings be upon him) said: "Whoever frees his share in a slave and has wealth that reaches the slave's price, the freeing of the rest of the slave is his responsibility."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب العتق / 21331