Sahih al-Bukhari 245
Sahih al-Bukhari · Book 4: Ablutions (Wudu') · 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
Narrated Hudhaifa:
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.
Whenever the Prophet (ﷺ) got up at night, he used to clean his mouth with Siwak.