Musnad Abu Ya'la al-Mawsili 161:62

Surayj ibn Yunus narrated to us, he said: Abu Hafs al-Abbar narrated to us, from Mansur ibn al-Mu'tamir, from Abu Ali, from Ja'far ibn Tamam, from his father, from al-Abbas, who said: They used to enter upon the Prophet (peace and blessings of Allah be upon him) and not use the siwak. So he said: "You enter upon me with yellowed teeth and do not use the siwak? Use the siwak. Were it not that I would burden my nation, I would have made the siwak obligatory upon them just as I made ablution obligatory upon them." And Aisha said: The Prophet (peace and blessings of Allah be upon him) did not cease mentioning the siwak until we feared that a Quran would be revealed regarding it.