Musnad Abu Ya'la al-Mawsili 42:409

From Abu Sa'id al-Khudri, from the Messenger of Allah (may Allah bless him and grant him peace), he said: "The earth eats everything of the human being except his tailbone." It was said: "And what is it like, O Messenger of Allah?" He said: "Like a mustard seed; from it they will grow."