Fiqh-us-Sunnah Volume 1, Purification and Prayer, Fiqh 1.014A.

Ibn ‘Abbas relates from Maimunah that the Prophet, upon whom be peace, was asked about a mouse that fell into a pot of clarified butter. He said, “Take (the mouse) and what is around it out, and throw it away. Then eat (the rest of) your clarified butter.” This is related by al-Bukhari.

Commenting on the subject, al-Hafezh Ibn Hajr says, “Ibn ‘Abdul Barr reported that there is agreement that if a dead animal falls into a solid matter, what the dead animal touches and what is around it must be thrown away, provided that one can make sure that the animal did not touch the remainder. As for a liquid substance, there is some difference of opinion. The majority say that the entire liquid becomes impure; az-Zuhri, al-Auza’i, and some others disagree with that opinion.