Ja'far bin 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 you come 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 ordered them to use the siwak at every prayer."
Hadith Referenceالمعجم الكبير للطبراني / باب التاء / 1290