Sahih al-Bukhari 7240
Sahih al-Bukhari · Book 94: Wishes · What uses of Al-Lau are allowed
Narrated Abu Huraira:
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 " .
Allah's Messenger (ﷺ) 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).