It is narrated from Sayyiduna Zayd bin Khalid al-Juhani (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) said: "Were it not that I would make things difficult for my Ummah, I would have commanded them to use the tooth-stick (siwak) at every prayer."
Hadith Referenceالمعجم الكبير للطبراني / باب الزاي / 5204