Messenger of Allah ﷺ said, "If I had not found it hard for my followers or the people, I would have ordered them to clean their teeth with Siwak for every prayer."  

أَنَّ رَسُولَ اللَّهِ ﷺ قَالَ لَوْلاَ أَنْ أَشُقَّ عَلَى أُمَّتِي أَوْ عَلَى النَّاسِ لأَمَرْتُهُمْ بِالسِّوَاكِ مَعَ كُلِّ صَلاَةٍ  

My father said, "I came to the Prophet ﷺ and saw him carrying a Siwak in his hand and cleansing his teeth, saying, 'U' U'," as if he was retching while the Siwak was in his mouth."  

أَتَيْتُ النَّبِيَّ ﷺ فَوَجَدْتُهُ يَسْتَنُّ بِسِوَاكٍ بِيَدِهِ يَقُولُ أُعْ أُعْ وَالسِّوَاكُ فِي فِيهِ كَأَنَّهُ يَتَهَوَّعُ  

Whenever the Prophet ﷺ got up at night, he used to clean his mouth with Siwak.  

كَانَ النَّبِيُّ ﷺ إِذَا قَامَ مِنَ اللَّيْلِ يَشُوصُ فَاهُ بِالسِّوَاكِ