Musannaf Ibn Abi Shayba 1:1644

1644. Abu Mu'awiyah narrated to us, from al-A'mash, from Abu Sufyan, from Jabir, who said: The Messenger of Allah (may Allah bless him and grant him peace) said: "If one of you cleans himself with stones (istajmar), let him clean himself with stones three times," meaning: let him perform istinja (cleaning the private parts).