Qutham bin Tammam, or perhaps Tammam bin Qutham, reports this statement of his father: We came to the Prophet (peace and blessings be upon him), and he (peace and blessings be upon him) said: Why is it that you have come to me like this (i.e. without cleaning your mouths)? Do you not use the siwak? Were it not that I feared I would burden my Ummah with hardship, I would have made the siwak obligatory upon them just as I have made ablution obligatory upon them.
Imam Ahmad recorded this report; its chain contains a narrator, Abu Ali al-Sayqal, about whom it has been said that he is majhul (unknown).
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الطهارة / 1120