Musnad Ahmad 27:1025

And Yazid informed us, he said: Yahya informed us, from Nafi’, from Ibn ‘Umar, that he used to say: The Messenger of Allah, may Allah bless him and grant him peace, said: "Whoever emancipates his share in a human being or a slave, he is obligated to emancipate the remainder of him. If he does not have the wealth to emancipate him with, then what has been emancipated is valid."