Sheikh Saaleh Al-Fawzaan (hafidhahullah)was asked:
‘If something from food remains between the teeth of the fasting person, is that considered from those things which break the fast if the fasting person swallows it? Benefit me may Allah bless you.’
And he replied:
‘If a fasting person wakes up and finds between his teeth something from the leftovers of food, then it is upon him to spit out these leftovers and to get rid of them. And it does not affect his fasting except if he swallows them intentionally, for indeed this ruins his fasting. As for if he swallows it out of ignorance or out of forgetfulness, then this does not affect his fasting. And it is imperative for the Muslim to strive to clean his mouth after (having) food, whether in the state of fasting or other than it, because cleanliness is requested from the Muslim and that he pays attention in cleaning his teeth and mouth after (having) food so that no leftovers remain in it whereby foul smells emanate from them, and by which it (the mouth) is harmed and by which it induces health-related harms upon his teeth.’
Source: Al-Muntaqa Min Fataawa Fadeelati-Sheikh Saaleh Bin FawzaanBin ‘Abdillah Al-Fawzaan, Vol.2, Pp 361-362,question 442.