Musnad Ahmad 30:2421

Yahya narrated to us, from Muhammad ibn ‘Amr, Abu Salamah narrated to us, from Abu Hurairah, 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 at every prayer, or with every prayer."