Musnad Ahmad 326:1

Yazid ibn Harun narrated to us, he said: Hajjaj ibn Artah narrated to us, from Amr ibn Shu'ayb, from Sa'id ibn al-Musayyib, who said: We memorized from thirty of the companions of the Messenger of Allah, peace and blessings of Allah be upon him, that he said: "Whoever emancipates a portion he owns of a slave, he is responsible for the remainder of him."