Maliks Muwatta Book 2, Hadith Number 116.
Section : The Tooth-stick (Siwak).
Yahya related to me from Malik from Abu’z Zinad from al-Araj from Abu Hurayra that the Messenger of Allah, may Allah bless him and grant him peace, said, “Were it not that I would be overburdening my community I would have ordered them to use a tooth-stick.”