Messenger of Allah ﷺ said, "Were I not afraid that it would be hard on my followers, I would order them to use the siwak (as obligatory, for cleaning the teeth). (Using translation from Bukhārī 7240)
أَنَّ رَسُولَ اللهِ ﷺ قَالَ «لَوْلَا أَنْ أَشُقَّ عَلَى أُمَّتِي لَأَمَرْتُهُمْ بِالسِّوَاكِ مع الوضوء»