Narrated Abu Huraira:
The Prophet (ﷺ) 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.
USC-MSA web (English) reference: Vol. 6, Book 60, Hadith 338
Arabic reference: Book 65, Hadith 4814