Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) says that the Messenger of Allah (peace and blessings be upon him) said: "Whoever frees his share of a slave shall be obliged to free the rest of the slave."
Yahya (may Allah have mercy on him) said: I do not know whether he said this from himself or whether it is in the hadith: if he does not have the price, then what he has done is sufficient.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب العتق / 21345