Musnad Abu Ya'la al-Mawsili 145:10

Ishaq ibn Abi Isra'il narrated to us, Abd al-Warith narrated to us, from Shu'ayb, from Anas, who said: The Messenger of Allah (may Allah bless him and grant him peace) said: "I have spoken to you at length regarding the siwak (tooth-stick)."