Musnad Ahmad 30:1164

'Ali ibn Hafs narrated to us, Waraqa informed us, from Abu al-Zinad, from al-A'raj, from Abu Hurayrah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Every son of Adam will be consumed by the earth, except for the coccyx (tailbone); for from it he was created, and from it he will be reconstructed."