Sunan an-Nasa'i 7
سنن النسائي#7SahihIt was narrated from Abu Hurairah that the Messenger of Allah (ﷺ) said:"Were it not that it would be too difficult for my Ummah, I would have commanded them to use the Siwak at (the time of) every Salah
Scholarly Authentication
We show the most cautious grade (weakest-wins) as the overall label. Individual scholars graded it as follows:
Abu GhuddahSahih
Al-AlbaniSahih
Zubair Ali ZaiSahih - Agreed Upon
In-book reference: 1:7