Musnad Ahmad 30:395

Abu Ubaydah al-Haddad, a trustworthy Kufan, narrated to us, from Muhammad ibn Amr, from Abu Salamah, 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 at every prayer to perform ablution—or with every ablution to use the toothstick (siwak)—and I would have delayed the final night prayer (Isha) until one-third of the night."