When the Prophet ﷺ got up to pray at night, he would brush his teeth with the siwak. (Using translation from Nasāʾī 1621)

«أَنَّ النَّبِيَّ ﷺ كَانَ إِذَا قَامَ مِنَ اللَّيْلِ يَشُوصُ فَاهُ»