Musannaf Ibn Abi Shayba 13:3388

Abu Bakr narrated to us, he said: Jarir narrated to us, from Mughira, from Ibrahim, who said: If a man says to his male slave or his female slave, "You are emancipated," "You are free," or "You are for Allah," then he is emancipated. And if he says, "You are my freedman (mawla)," then he is emancipated.