Musnad Ahmad 33:438

Abu Ahmad narrated to us, Sufyan narrated to us, from Abu al-Zubayr, from Jabir, who said: The Messenger of Allah (peace and blessings of Allah be upon him) departed while tranquility was upon him, and he quickened his pace in the valley of Muhassir. He showed them [the size of the pebbles] like the pebbles used for throwing (al-khadhf), and he commanded them to maintain tranquility. He said: "Let my nation take their rites [of Hajj], for I do not know, perhaps I will not meet them after this year of mine."