If you take water into your mouth for some reasons while fasting, how can you make sure that the remaining water in your mouth doesn't break your fast?
Just try avoiding drink the remained water as far as you can. If you did, so the little quantity doesn't break your fast insha'Allah
IF you don't drink or eat anything intentionally your fast will not break.Sahih
Al-Bukhari Hadith 8.662 Narrated by Abu Huraira
The Prophet (saws) said, "If somebody eats something forgetfully while he is fasting, then he should complete his fast, for Allah has made him eat and drink."