Whatever is cut from an animal while it is alive, then it is dead flesh
Authentic hadithعن أبي واقد الليثي رضي الله عنه قال: قَدِم النبي صلى الله عليه وسلم المدينة وهم يَجُبُّون أَسْنِمة الإبل، ويَقْطعون أَلْيَات الغنم، فقال: «ما قُطِع من البَهِيمَة وهي حيَّة فهي ميْتَة».
Abu Wāqid al-Laithi (may Allah be pleased with him) reported: The Prophet (may Allah's peace and blessings be upon him) came to Madīnah and they were in the habit of cutting the humps off the camels and cutting the buttocks from the sheep. He said: "Whatever is cut from an animal while it is alive, then it is dead flesh."
Explanation
This Hadīth relates that the Prophet (may Allah's peace and blessings be upon him) came to Madīnah and found its people in the habit of cutting off the camels' humps and sheep's buttocks, which they would eat and utilize. So, he forbade them from doing this and clarified the rule in this regard; that if anything is cut off a living animal, like a camel's hump or sheep's buttocks, by itself or someone, it will be treated like a dead animal of the same kind; if it is deemed pure, this piece will be regarded as such, and vice versa. The hand of a human being is pure, while the buttocks of sheep is impure. Excluded are the hair, wool, feather, and skin.
Source · Narrated by At-Termedhy - Narrated by Abu Daoud - Narrated by Ahmad - Ad-Daarimi
From the Hadith Encyclopedia (hadeethenc.com).