The Messenger of Allah ﷺ said, "The earth consumes every part of a human being except the tailbone."Someone asked, "What is it like, Messenger of Allah?"He replied, "It is like a mustard seed. From it you will be reconstituted."
قال رسول الله ﷺ: يأكل التراب كل شيء من الإنسان إلا عجب ذنبه قيل: مثل ما هو يا رسول الله؟ قال: مثل حبة خردل منه تنبتون.
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Ahmad (11230) and Abu Yaala (1382) both narrated it through the route of Hasan bin Musa, from Ibn Lahia, from Darraj Abu Samh, from Abu Haytham, from Abu Saeed.Darraj Abu Samh is disputed. The majority hold him to be daif, especially in his narration from Abu Haytham, from Abu Saeed, as Abu Dawud said. However, Ibn Adi said, "Most of the hadiths I have dictated from Darraj are ones for which there is no corroboration," but he did not mention this hadith among those he considered objectionable, and he said, "I hope that his hadiths after those I have criticized are not problematic."As for Ibn Lahia, he has been corroborated in Ibn Hibban's Sahih (3140) and by Hakim (4/609).