Fiqh-us-Sunnah Volume 3, Zakaat and Fasting, Fiqh 3.133.

Section : Fasting, using the tooth stick [brush].

It is preferred for the fasting person to use a tooth stick or a brush. There is no difference if he uses it at the beginning or the ending of the day. At-Tirmizhi affirms that: “Ash-Shafhi did not see anything wrong with using a tooth stick [brush] during the beginning or the ending of the day.” The Prophet would use his tooth stick [brush] while fasting.