It is narrated from Sayyiduna Abdullah bin al-Zubayr (may Allah be pleased with them both) that the Messenger of Allah (peace and blessings be upon him) said: "Were it not that I feared hardship for my Ummah, I would have ordered them to use the siwak (tooth-stick) at every prayer."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 13923