Musnad Ahmad 737:2

'Affan narrated to us, Wuhayb narrated to us, 'Abd al-Rahman ibn Harmalah narrated to us, from Yahya ibn Hind, that he heard Harmalah ibn 'Amr—who is Abu 'Abd al-Rahman—say: I performed the Farewell Pilgrimage, with my uncle Sinan ibn Sunnah riding behind me. He said: When we stood at 'Arafat, I saw the Messenger of Allah, peace and blessings of Allah be upon him, placing one of his fingers upon the other. I said to my uncle: What is the Messenger of Allah, peace and blessings of Allah be upon him, saying? He said: He is saying: "Throw the Jamrah with pebbles the size of a khadhf (a small pebble thrown between the thumb and index finger)."