It is narrated from Sayyiduna al-Hirmas bin Ziyad (may Allah be pleased with him); he said: I saw the Prophet (peace and blessings be upon him) while I was riding behind my father; he (peace and blessings be upon him) was on his she-camel al-Adba' on the Day of Sacrifice, and the people were around him. I asked my father: What is the Messenger of Allah (peace and blessings be upon him) saying? He said: He (peace and blessings be upon him) is saying: "Stone the Jamarat with pebbles like khadhf pebbles (that is, small pebbles flicked with the fingers)."
Hadith Referenceالمعجم الكبير للطبراني / باب الهاء / 18753