Musnad Ahmad 30:3346

'Amr ibn Mujamma' Abu al-Mundhir al-Kindi narrated to us, Ibrahim al-Hajari narrated to us, from Abu 'Iyad, from Abu Hurairah, who said: The Messenger of Allah, peace and blessings of Allah be upon him, said: "Every bone of the son of Adam will decay, except for the coccyx (tailbone), and from it the creation will be assembled on the Day of Resurrection."