Ja'far bin Tamim bin al-Abbas, or Ibn Tammam bin al-Abbas, narrates from his father, Sayyiduna Tammam bin al-Abbas (may Allah be pleased with him), who said that the Messenger of Allah (peace and blessings be upon him) said: "Why is it that I see you coming to me with yellowed teeth? Use the tooth-stick (siwak). Were it not that I would make things hard for my Ummah, I would have made the siwak obligatory upon them just as the prayer has been made obligatory upon them."
Hadith Referenceالمعجم الكبير للطبراني / باب التاء / 1288