Musannaf Ibn Abi Shayba 16:170

Abu Bakr narrated to us, he said: Zayd ibn al-Hubab narrated to us, from Humayd ibn Ubayd al-Ta’i, he said: Thumamah ibn Abd Allah ibn Anas narrated to me, from his grandfather Anas, that he was asked about a mouse that falls into ghee and oil. He said: "If it is solid, it (the mouse) and what is around it should be taken and thrown away, and what remains should be eaten. If it is liquid, use it for lighting."