Musnad Ahmad 30:294

Yahya narrated to us, Ubaydullah informed us, Sa'id ibn Abi Sa'id narrated to me, 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 to use the siwak with every ablution, and I would have delayed the Isha prayer until a third of the night, or half of the night."