Sayyiduna Harmalah bin Amr (may Allah be pleased with him) narrates: I performed the Farewell Hajj with the Prophet (peace and blessings be upon him), and I was a boy at the time, riding behind my uncle. I saw the Messenger of Allah (peace and blessings be upon him) placing one of his fingers upon the other, so I asked my uncle: What is he saying? He said: He is saying: "Throw pebbles the size of a chickpea."
Hadith Referenceالمعجم الكبير للطبراني / باب الحاء / 3460