Sunan at-Tirmithiy كتاب الطهارة باب ما جاء فى السواك
The Messenger of God, peace and blessings of God be upon him said, "Were it not for the hardship I would cause my nation, I would have ordered them to use siwāk (natural toothbrush) before each prayer, and I would have delayed the evening prayer (1) until one third of the night had passed." (1) Fifth and last prescribed prayer of the day. Click on ḥadīth to read more