Musnad Ahmad 23:1

Isma'il ibn 'Umar Abu al-Mundhir narrated to us, he said: Sufyan narrated to us, from Abu 'Ali al-Zarrad, he said: Ja'far ibn Tamam ibn 'Abbas narrated to me, from his father, he said: They came to the Prophet, may Allah bless him and grant him peace—or he was brought—and he said: "Why do I see you coming to me with yellow teeth? Use the siwak (tooth stick). 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."