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Al-Adab al-Mufrad 736
حَدَّثَنَا ابْنُ نُمَيْرٍ قَالَ: حَدَّثَنِي أَبِي، قَالَ: حَدَّثَنَا إِسْمَاعِيلُ، عَنْ قَيْسٍ قَالَ: كَانَ عَمْرُو بْنُ الْعَاصِ يَسِيرُ مَعَ نَفَرٍ مِنْ أَصْحَابِهِ، فَمَرَّ عَلَى بَغْلٍ مَيِّتٍ قَدِ انْتَفَخَ، فَقَالَ: وَاللَّهِ، لَأَنْ يَأْكُلَ أَحَدُكُمْ هَذَا حَتَّى يَمْلَأَ بَطْنَهُ، خَيْرٌ مِنْ أَنْ يَأْكُلَ لَحْمَ مُسْلِمٍ.
It is reported by Qays that Amr bin al-Aas (ra) was going somewhere with his friends. They saw a dead mule whose belly had swollen up. Amr (ra)said," By Allah, it is better for one to eat to his full from this (dead mule) than to eat the flesh of a Muslim" (He meant to say that to backbite anyone is worse than eating the meat of a dead animal).
Cited in Al-Adab al-Mufrad (No. 736).