Musnad Ahmad 5:46

Abdullah narrated to us, Uqbah ibn Mukram al-Kufi narrated to me, Yunus ibn Bukayr narrated to us, Muhammad ibn Ishaq narrated to us, from Sa’id ibn Abi Sa’id al-Maqburi, from Abu Hurayrah, and from Ubayd Allah ibn Abi Rafi’, from his father, from Ali, they both said: The Messenger of Allah, may Allah bless him and grant him peace, said: "Were it not that I would make it difficult for my nation, I would have commanded them to use the siwak (tooth-stick) at every prayer."