narrated abu huraira (radi-allahu 'anhu):

the prophet (sallallahu 'alaihi wa sallam) said, "if somebody eats or drinks forgetfully then he should complete his fast, for what he has eaten or drunk, has been given to him by allah." narrated 'amir bin rabi'a, "i saw the prophet (sallallahu 'alaihi wa sallam) cleaning his teeth with siwak while he was fasting so many times as i can't count." and narrated abu huraira, "the prophet (sallallahu 'alaihi wa sallam) said, 'but for my
fear that it would be hard for my followers, i would have ordered them to clean their teeth with siwak on every performance of ablution." the same is narrated by jabir and zaid bin khalid from the prophet (sallallahu 'alaihi
wa sallam) who did not differentiate between a fasting and a nonfasting person in this respect (using siwak).

aisha  (radi-allahu 'anha) said, "the prophet (sallallahu 'alaihi wa sallam) said, "it (i.e. siwak) is a purification for the mouth and it is a way of seeking allah's pleasures." ata' and qatada said, "there is no harm in swallowing the resultant saliva."

bukhari vol. 3 : no. 154