Musannaf Ibn Abi Shayba 13:3386

Abu Bakr narrated to us, he said: Abu Usamah narrated to us, from Hisham, from al-Hasan, regarding a man who says: "If I buy such-and-such woman, then she is free," or "Every slave girl I buy from you is free." He said: If he buys anything of that, then she has been emancipated.