Hudhayfah reported that whenever the Prophet ﷺ rose at night for the tahajjud prayer, he would scrub his teeth thoroughly with the siwak.
أن النبي ﷺ كان إذا قام للتهجد من الليل يشوص فاه بالسواك.
Bukhari narrated it in Tahajjud (1136), and Muslim in Purification (255), both from the hadith of Husayn, from Abu Waail, from Huzaifa, who mentioned it.