Musnad Ahmad 1208:1

Yazid ibn Harun narrated to us, he said: Al-Hajjaj ibn Artah narrated to us, from Abu Yazid, the freed slave of Abdullah ibn al-Harith, from Umm Jundub al-Azdiya, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "O people, do not kill one another at Jamrat al-Aqaba, and you should use [pebbles] the size of the pebbles used for throwing (al-khadhf)."