Al-Kāfi - Volume 6 > The Manner of Slaughtering and al-Nahr
Hadith #8

مُحَمَّدُ بْنُ يَحْيَى رَفَعَهُ قَالَ قَالَ أَبُو الْحَسَنِ الرِّضَا ( عليه السلام ) إِذَا ذُبِحَتِ الشَّاةُ وَ سُلِخَتْ أَوْ سُلِخَ شَيْ‏ءٌ مِنْهَا قَبْلَ أَنْ تَمُوتَ لَمْ يَحِلَّ أَكْلُهَا .

8. Muhammad ibn Yahya has narrated from in a marfu‘ manner the following: “Abu al-Hassan, al-Rida’, ‘Alayhi al-Salam, has said, ‘When you slaughter a sheep and remove its skin or a certain part of its skin before it dies, it then is not lawful to use it for food.’”