The earth would consume all of the son of Adam except his tailbone. From it he was created, and from it he will be recreated (on the Day of Resurrection).
Sahih Muslim Book 54, Hadith 176
← Previous Hadith Between the two blowings of the trumpet (there would be an interval of forty). They said: Abu Huraira, do you mean forty days? He said: I cannot say anything. They said: Do you mean forty months? He said: I cannot say anything. They said: Do you mean forty years? He said: I cannot say anything. Then Allah would cause the water to, descend from the sky and they (people) will sprout like vegetable. The only thing in a man which would not decay would be one bone (the tailbone) from which the whole frame would be reconstituted on the Day of Resurrection.

Sahih Muslim Book 54, Hadith 175 Next Hadith → There is a bone in the human being which the earth would never consume and it is from this that new bodies would be reconstituted (on the Day of Resurrection). They said: Allah's Messenger, which bone is that? Thereupon he said: It is the spinal bone.

Sahih Muslim Book 54, Hadith 177