Musnad Abu Ya'la al-Mawsili 159:43

Bishr narrated to us, Abd al-Rahman narrated to us, from his father, from al-A'raj, from Abu Hurayrah, who said: The Messenger of Allah (may Allah bless him and grant him peace) said: "Every son of Adam will be consumed by the earth, except for the coccyx (tailbone). From it he was created, and upon it he will be reconstructed."