Wa `alaykum As-Salamu wa Rahmatullahi wa Barakatuh.
In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
Dear brother in Islam, it gives us great pleasure to receive your question. May Allah bless your efforts in pursuit of knowledge and accept your fasting and all other acts of worship!
As regards your question, the eminent Muslim scholar, Sheikh Muhammad Al-Hanooti, member of the North American Fiqh Council, states:
"Rinsing the mouth and the nose during wudu' (ablution) is either classified as a Sunnah act of the Prophet (peace and blessings be upon him), according to Abu Hanifah, Malik, and Ash-Shafi`i, or as an obligation, according to Imam Ahmad who considers it to be a part of washing the face. In both cases, one should not abandon rinsing them when making wudu' whether fasting or not.
It is noteworthy that a fasting person should not exaggerate in doing it. Here, we recall the Prophetic hadith that reads: “When you rinse your nose, overdo it except if you are observing fasting.”
Accordingly, if one rinses his mouth or nose in wudu' while fasting and some water goes down his throat without intending or overdoing it, his Fast remains valid. This is similar to the case when dust from the roadway or a fly enters the throat, which is regarded as a mistake, for which this nation will not be held accountable, even if some scholars differ regarding them.
Also, rinsing the mouth for other aims than wudu' will not invalidate one's Fasting as long as none of the water goes down the throat."
