Sayyiduna Abu Hurayrah (may Allah be pleased with him) narrates from the Prophet (peace and blessings be upon him): "Whoever has a share in a slave and frees it, it is binding upon him to free the rest, if he has wealth; and if he has no wealth, the slave is to be made to work (to earn the rest of his price), but without being overburdened."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب العتق / 21369