It was narrated from Abu Hurairah that the Messenger of Allah (ﷺ) 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." (FR)
English reference (FR): Vol. 1, Book 6, Hadith 535 (FR)
Arabic reference (FR): Book 6, Hadith 539 (FR)