The Messenger of Allah ﷺ said, "I have urged you repeatedly concerning the use of the siwak."
قال رسول الله ﷺ: لقد أكثرت عليكم في السواك.
No references are available.
Bukhari reported it in the chapter on Friday (888), from Abu Muamar, who said, Abdul-Warith narrated to us, who said, Shuayb bin Habhab narrated to us, Anas narrated to us... and he mentioned it.