Musnad Ahmad 33:866

Abu Dawud narrated to us, Rabah al-Makki narrated to us, from Abu al-Zubayr, from Jabir ibn Abdullah: "That the Prophet, peace and blessings of Allah be upon him, commanded them to throw the pebbles like the pebbles of al-khadhf (throwing a small pebble by placing it between the two index fingers)."