Sunan an-Nasa'i, Volume 2, Book 20, Hadith 1623 (DarusSalam) ⓘ
It was narrated that Hudhaifah said: "When the Messenger of Allah got up to pray tahajjud at night, he would brush his teeth with the siwak.
حَدَّثَنَا مُحَمَّدُ بْنُ عَبْدِ الأَعْلَى، قَالَ حَدَّثَنَا خَالِدٌ، قَالَ حَدَّثَنَا شُعْبَةُ، عَنْ حُصَيْنٍ، قَالَ سَمِعْتُ أَبَا وَائِلٍ، يُحَدِّثُ عَنْ حُذَيْفَةَ، قَالَ كَانَ رَسُولُ اللَّهِ صلى الله عليه وسلم إِذَا قَامَ مِنَ اللَّيْلِ يَشُوصُ فَاهُ بِالسِّوَاكِ .
also referenced under: Hadith 1622 (Reference) & Book 20, Hadith 25 (In-Book)