Musannaf Ibn Abi Shayba 7:366

Hushaym narrated to us, from Mughirah, from Ibrahim, that he used to say: If one swears an oath regarding milk, he should not eat butter, for it is from milk. If one swears an oath regarding butter, he may eat milk. If one swears an oath regarding meat, he should not eat fat, for it is from meat. If one swears an oath regarding fat, he may eat meat.