Musnad Ahmad 33:1179

Ali ibn Bahr narrated to us, Isa ibn Yunus narrated to us, from al-A’mash, from Abu Sufyan, from Jabir, the like of it. He said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "If one of you cleans himself with stones (istajmar), let him clean himself with stones three times."