Sahih al-Bukhari 4935

Muhammad related to me; Abu Muawiya informed us, from al-Amash, from Abu Salih, from Abu Hurayra (may Allah be pleased with him), who said: The Messenger of Allah (peace and blessings be upon him) said, ‘Between the two blasts there will be forty.’ He was asked, ‘Forty days?’ He replied, ‘I refuse to say.’ He was asked, ‘Forty months?’ He replied, ‘I refuse to say.’ He was asked, ‘Forty years?’ He replied, ‘I refuse to say.’ Then he said: Allah will send down water from the sky, and they will grow as vegetation grows. Nothing of the human being will decay except one bone, the coccyx; from it the creation will be assembled on the Day of Resurrection.’