Language:
كتاب الوضوء
4  
Ablutions (Wudu')
(73)
Chapter: Siwak (to clean the teeth with Siwak which is a tooth-brush in the form of a pencil from the roots of the Arak tree
(73)
باب السِّوَاكِ
Sahih al-Bukhari 245

Narrated Hudhaifa:

Whenever the Prophet (ﷺ) got up at night, he used to clean his mouth with Siwak.

حَدَّثَنَا عُثْمَانُ، قَالَ حَدَّثَنَا جَرِيرٌ، عَنْ مَنْصُورٍ، عَنْ أَبِي وَائِلٍ، عَنْ حُذَيْفَةَ، قَالَ كَانَ النَّبِيُّ صلى الله عليه وسلم إِذَا قَامَ مِنَ اللَّيْلِ يَشُوصُ فَاهُ بِالسِّوَاكِ‏.‏
Reference : Sahih al-Bukhari 245
In-book reference : Book 4, Hadith 111
USC-MSA web (English) reference : Vol. 1, Book 4, Hadith 246
  (deprecated numbering scheme)