Maliks Muwatta Book 2, Hadith Number 117.
Section : The Tooth-stick (Siwak).
Yahya related to me from Malik from Ibn Shihab from Humayd ibn Abdar-Rahman ibn Awf that Abu Hurayra said, “Were it not that he would be overburdening his community he (the Messenger of Allah, may Allah bless him and grant him peace,) would have ordered them to use a tooth-stick with each wudu.”