Sahih Bukhari Volume 6, Book 60, Hadith Number 338.

Narrated By Abu Huraira : The Prophet said, “Between the two blowing of the trumpet there will be forty.” The people said, “O Abu Huraira! Forty days?” I refused to reply. They said, “Forty years?” I refused to reply and added: Everything of the human body will decay except the coccyx bone (of the tail) and from that bone Allah will reconstruct the whole body.