Sayyiduna Abu Sa'id al-Khudri (may Allah be pleased with him) reports that the Prophet (peace and blessings be upon him) said: "The soil consumes the whole human being except for one particular part in the back (the tailbone). It was asked: O Messenger of Allah, what is it like? The Prophet (peace and blessings be upon him) said: It is like a mustard seed; from it you will be raised again."
Ahmad recorded it, and its chain is hasan (good).
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب البعث / 18314