Musannaf Ibn Abi Shayba 13:826

Abu Bakr narrated to us, he said: Abbad bin al-Awwam narrated to us, from Harun bin Musa, he said: Ali bin Za'idah informed me, from Nafi', from Ibn Umar, that he was asked about a man who buys a slave girl, and her master stipulates her emancipation. He said: The reward is for her master who stipulated it.