Musnad Ahmad 137:1

Mu'awiyah ibn Hisham narrated to us, he said: Sufyan narrated to us, from Abu Ali al-Sayqal, from Qutham ibn Tammam or Tammam ibn Qutham, from his father, he said: We came to the Prophet, may Allah bless him and grant him peace, and he said: "What is the matter with you that you come to me with yellow teeth, not using the siwak? Were it not that it would be difficult for my nation, I would have made the siwak obligatory upon them, just as I made ablution obligatory upon them."