Mishkat al-Masabih 3 Purification كتاب الطهارة 88

وَعَنْ حُذَيْفَةَ قَالَ: كَانَ النَّبِيُّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ إِذَا قَامَ لِلتَّهَجُّدِ مِنَ اللَّيْلِ يَشُوصُ فَاهُ بِالسِّوَاكِ

Hudhaifa said that when the Prophet got up to pray during the night he cleansed his mouth with the tooth-stick. (Bukhari and Muslim.)