It is narrated from Sayyiduna Wathilah bin al-Asqa' (may Allah be pleased with him) that he said: The Messenger of Allah (peace and blessings be upon him) said: "I have been commanded to use the siwak (tooth-stick) so much that I feared it would be made obligatory upon me."
Hadith Referenceالمعجم الكبير للطبراني / باب الواو / 18410