Musnad Abu Ya'la al-Mawsili 159:22

Abu Khaythama narrated to us, Sufyan narrated to us, from Abu al-Zinad, from al-A'raj, from Abu Hurayrah, from the Prophet (may Allah bless him and grant him peace), who said: "Were it not that I would make it difficult for my nation, I would have commanded them to delay the Isha prayer, and I would have commanded them to use the siwak at every prayer."