Musnad Ahmad 1208:2

Hashim narrated to us, he said: Layth informed us, from Abdullah ibn Shaddad, from Umm Jundub al-Azdiya, that she heard the Prophet, peace and blessings of Allah be upon him, when he departed [from Muzdalifah], say: "O people, upon you is tranquility and dignity, and upon you is [to use] the likes of the pebbles of al-khadhf [small pebbles]."