It was narrated from Abu Hurairah (RA) that the Messenger of Allah
(peace be upon him) said: “Were it not that I would impose too much
difficulty on my Ummah, I would have commanded them to delay ‘Isha’ and
to use the Siwak for every prayer.”
(Sunan an-Nasa’i, Book 6, Hadith 539)
source:HadithOfTheDay.com