Musnad Ahmad 30:2800

I read to Abd al-Rahman: Malik, from Ibn Shihab, from Humayd ibn Abd al-Rahman ibn Awf, from Abu Hurayrah, from the Messenger of Allah, may Allah bless him and grant him peace, that he said: "Were it not that it would be difficult for my nation, I would have commanded them to use the siwak with every ablution."