Narrated Abu Huraira (rad):

Allah’s Messenger (ﷺ) said “Had I not feared burdening my Ummah, I would have commanded them to use Siwak with every ablution.” [Reported by Malik, Ahmad and An-Nasai. Ibn Khuzaima graded it Sahih (sound) and Al-Bukhari mentioned as a Mu’allaq (suspended – without its chain of narrators).

عَنْ أَبِي هُرَيْرَةَ ‏- رضى الله عنه ‏- عَنْ رَسُولِ اَللَّهِ ‏- صلى الله عليه وسلم ‏-قَالَ: { لَوْلَا أَنْ أَشُقَّ عَلَى أُمَّتِي لَأَمَرْتُهُمْ بِالسِّوَاكِ مَعَ كُلِّ وُضُوءٍ } أَخْرَجَهُ مَالِكٌ, وأَحْمَدُ, وَالنَّسَائِيُّ, وَصَحَّحَهُ اِبْنُ خُزَيْمَة َ [1]‏ .‏