We came to the Prophet ﷺ, and he said, "Why do you come to me with yellowed teeth? Use the siwak. Were it not that I would burden my Ummah, I would have made the siwak obligatory upon them, just as I made wudu obligatory upon them."
حدثنا معاوية بن هشام قال حدثنا سفيان عن أبي علي الصيقل
أتينا النبي ﷺ فقال ما بالكم تأتوني قلحا لا تسوكون لولا أن أشق على أمتي لفرضت عليهم السواك كما فرضت عليهم الوضوء
Its chain of transmission is daif, due to inconsistency in its chain. Abu Ali the Sayqal here at one time narrates it from Qutham bin Tammam from his father, and at another time narrates it from Tammam bin Qutham from his father. In sources other than Ahmad, he narrates it from Jafar bin Tammam bin Abbas from his father from his grandfather, as found in Abu Hanifa and Tabarani.Even if the chain were sound, Haythami said about Abu Ali the Sayqal in 2/97 and 1/2121 that he is majhul (unknown), and likewise in Tajil.The hadith was narrated by Abu Hanifa in 1/421 of Jami al-Masanid from Tammam from Jafar bin Abu Talib, and also from Jafar bin Tammam from his father from Abbas.The Sayqal was identified as Ali bin Jafar bin Muhammad bin Abdullah bin Ali the Sayqal, but I also could not find a biography for him. I also failed to search for him among the teachers of Abu Hanifa, all of whom were narrated from, in the book Abu Hanifa Between the People of Criticism and Validation.It was likewise narrated by Tabarani in Al-Kabir 2/94, numbers 1301-1302-1303, similarly to Abu Hanifa.