Whenever the Messenger of Allah ﷺ rose at night, he would clean his mouth with the siwak.
أخبرنا إسحق بن إبراهيم وقتيبة بن سعيد عن جرير عن منصور عن أبي وائل
كان رسول الله ﷺ إذا قام من الليل يشوص فاه بالسواك
[Agreed upon by Bukhari, Book of Wudu, Chapter on the Tooth-stick, no. (245), and others, and Muslim, no. (255), from the hadith of Jareer bin Abdul-Hameed, from Mansur bin Mutamir, with this wording. It is also found in Kubra, Book of Purification, Chapter on the Tooth-stick, no. (2)].