Musnad Ahmad 30:2072

Abu al-Ala’ al-Hasan ibn Siwar narrated to us, he said: Layth narrated to us, from Khalid ibn Yazid, from Sa’id ibn Abi Hilal, from Abd al-Rahman al-A’raj, from Abu Hurayrah, that the Messenger of Allah (may Allah bless him and grant him peace) said—if he indeed said it—: "Were it not that I would make it difficult for my nation, I would have commanded them to use the siwak with every ablution." And Abu Hurayrah said: "I used to use the siwak before I slept, and after I woke up, and before I ate, and after I ate, when I heard the Messenger of Allah (may Allah bless him and grant him peace) say what he said."