Hadith 15656
It is narrated from Sayyiduna Qutham or Tamam that once we were present in the service of the Prophet (peace be upon him), and the Prophet (peace be upon him) said: What is the matter, why do I see your teeth yellow and discolored? Do you not use the miswak? If it were not difficult for my ummah, I would have made the use of miswak obligatory for them just as wudu is obligatory.