When the Prophet ﷺ got up to pray at night, he would brush his teeth with the siwak.  

أَنَّ النَّبِيَّ ﷺ كَانَ إِذَا قَامَ مِنَ اللَّيْلِ يَشُوصُ فَاهُ بِالسِّوَاكِ  

"When the Messenger of Allah ﷺ got up to pray tahajjud at night, he would brush his teeth with the siwak.  

كَانَ رَسُولُ اللَّهِ ﷺ إِذَا قَامَ مِنَ اللَّيْلِ يَشُوصُ فَاهُ بِالسِّوَاكِ