Musnad Ahmad 30:2401

Yahya narrated to us, from Ibn ‘Ajlan, who said: Abu al-Zinad narrated to us, from al-A‘raj, from Abu Hurayrah, from the Prophet, peace and blessings of Allah be upon him, that he said: "Every son of Adam will decay, and the earth will consume him, except for the coccyx (tailbone); from it he was created, and in it he will be reconstructed."