It is narrated from Sayyiduna Ibn Abbas (may Allah be pleased with them both) that the Messenger of Allah (peace and blessings be upon him) said: "I see you coming to me in a state where there is grime on your teeth. Had I not considered it a hardship for my Ummah, I would have made the siwak obligatory upon them just as ablution has been made obligatory upon them."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 151