Allah's Apostle 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."
Sahih Bukhari: Volume 2, Book 13, Number 12
Allah's Apostle said, "If anyone of you enters a mosque, he should pray two Rakat before sitting."
Sahih Bukhari: Volume 1, Book 8, Number 435