Sahih al-Bukhari 888
Sahih al-Bukhari · Book 11: Friday Prayer · To clean the teeth with Siwak on Friday
Narrated Anas:
حَدَّثَنَا أَبُو مَعْمَرٍ ، قَالَ حَدَّثَنَا عَبْدُ الْوَارِثِ ، قَالَ حَدَّثَنَا شُعَيْبُ بْنُ الْحَبْحَابِ ، حَدَّثَنَا أَنَسٌ ، قَالَ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم " أَكْثَرْتُ عَلَيْكُمْ فِي السِّوَاكِ " .
Allah's Messenger (ﷺ) said, "I have told you repeatedly to use the Siwak. (The Prophet (ﷺ) put emphasis on the use of the Siwak.)