Musannaf Ibn Abi Shayba 1:1797

Ubaydah ibn Humayd narrated to us, he said: Al-A’mash narrated to us, from Abdullah ibn Yasar, from Abd al-Rahman ibn Abi Layla, from some of the companions of the Prophet (may Allah bless him and grant him peace), who attributed it to him (the Prophet), he said: "Were it not that it would be difficult for my nation, I would have made the Siwak (tooth-stick) obligatory upon my nation, just as I made purification (Wudu) obligatory upon them."