Musannaf Ibn Abi Shayba 1:1787

Abu Usamah and Ibn Numayr narrated to us, from Ubaydullah ibn Umar, from Sa'id ibn Abi Sa'id, from Abu Hurayrah, who 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 at every ablution."