ʿAbdullah said, "One night I offered the Tahajjud prayer with the Prophet ﷺ and he kept on standing till an ill-thought came to me." We said, "What was the ill-thought?" He said, "It was to sit down and leave the Prophet (standing)."  

صَلَّيْتُ مَعَ النَّبِيِّ ﷺ لَيْلَةً فَلَمْ يَزَلْ قَائِمًا حَتَّى هَمَمْتُ بِأَمْرِ سَوْءٍ قُلْنَا وَمَا هَمَمْتَ قَالَ هَمَمْتَ أَنْ أَقْعُدَ وَأَذَرَ النَّبِيَّ ﷺ  

Whenever the Prophet ﷺ got up for Tahajjud prayer he used to clean his mouth (and teeth) with Siwak.  

أَنَّ النَّبِيَّ ﷺ كَانَ إِذَا قَامَ لِلتَّهَجُّدِ مِنَ اللَّيْلِ يَشُوصُ فَاهُ بِالسِّوَاكِ