Sahih al-Bukhari, Hadith 888 ⓘ
Narrated Anas: Allah's Messenger said, "I have told you repeatedly to use the Siwak. (The Prophet put emphasis on the use of the Siwak.)
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 " .
also referenced under: Book 11, Hadith 13 (In-Book) & Volume 2, Book 13, Hadith 13 (USC-MSA)