It 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."
أَخْبَرَنَا قُتَيْبَةُ بْنُ سَعِيدٍ، عَنْ مَالِكٍ، عَنْ أَبِي الزِّنَادِ، عَنِ الأَعْرَجِ، عَنْ أَبِي هُرَيْرَةَ، أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم قَالَ ‏ "‏ لَوْلاَ أَنْ أَشُقَّ عَلَى أُمَّتِي لأَمَرْتُهُمْ بِالسِّوَاكِ عِنْدَ كُلِّ صَلاَةٍ ‏"‏ ‏.‏
GradeSahih (Darussalam)
Arabic/English book reference : Vol. 1, Book 1, Hadith 7