Sahih al-Bukhari, Volume 9, Book 90, Hadith 346 (USC-MSA) ⓘ
Narrated Abu Huraira: Allah's Messenger said, "Were I not afraid that it would be hard on my followers, I would order them to use the siwak (as obligatory, for cleaning the teeth).
حَدَّثَنَا يَحْيَى بْنُ بُكَيْرٍ، حَدَّثَنَا اللَّيْثُ، عَنْ جَعْفَرِ بْنِ رَبِيعَةَ، عَنْ عَبْدِ الرَّحْمَنِ، سَمِعْتُ أَبَا هُرَيْرَةَ ـ رضى الله عنه ـ أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم قَالَ " لَوْلاَ أَنْ أَشُقَّ عَلَى أُمَّتِي لأَمَرْتُهُمْ بِالسِّوَاكِ " .
also referenced under: Book 94, Hadith 15 (In-Book) & Hadith 7240 (DarusSalam)