Narrated Hudhaifa(RA) :
Whenever the Prophet(pbuh) got up for Tahajjud prayer he used to clean his mouth (and teeth) with Siwak. Volume 2, Book 21, Number 237: Sahih Bukhari
Abu Hurayrah (RA), who reported that the Prophet (peace and blessings of Allaah be upon him) said: “Were it not for the fact that I did not want to make things too hard for my ummah, I would have commanded them to use the siwaak at every time of prayer.” (Reported by al-Bukhaari, 2/299 and Muslim, 1/151).
Allah knows the best.