Musnad Ahmad 32:1851

'Abd al-Samad ibn Hassan narrated to us, 'Amarah informed us, from Thabit, from Anas, that the Messenger of Allah (peace and blessings of Allah be upon him) took three pebbles. He placed one down, then he placed another in front of him, and he threw the third one. He said: "This is the son of Adam, and this is his appointed time of death, and that is his hope," referring to the one he threw.