The Messenger of Allah ﷺ instructed us to check the eyes and ears, and not to sacrifice any animal whose ears are slit in two lengthwise, any animal that has a round hole in its ear as a distinguishing mark, any animal that has the edge of its ears cut, or any animal whose ears were slit from the back. (Using translation from Aḥmad 1061)  

«أَمَرَنَا رَسُولُ اللهِ ﷺ أَنْ نَسْتَشْرِفَ الْعَيْنَ وَالْأُذُنَ وَأَنْ لَا نُضَحِّيَ بِعَوْرَاءَ وَلَا مُقَابَلَةٍ وَلَا مُدَابَرَةٍ وَلَا شَرْقَاءَ وَلَا خَرْقَاءَ»