Musnad Ahmad 30:3565

Rawh narrated to us, Malik ibn Anas narrated to us, from Ibn Shihab, from Humayd ibn Abd al-Rahman, from Abu Hurayrah, who said: The Messenger of Allah, peace and blessings of Allah be upon him, said: "Were it not that I would make it difficult for my nation, I would have commanded them to use the siwak with every ablution."