Praise be to Allah.
The fasting person is forbidden to go to extremes in rinsing his nose, because the Prophet (peace and blessings of Allaah be upon him) said to Laqeet ibn Sabirah (may Allaah be pleased with him): “Do wudoo’ well, make the water run in between your fingers and toes, and go to extremes in rinsing the nose, unless you are fasting.” Narrated by Abu Dawood (142) and al-Tirmidhi (788); classed as saheeh by al-Albaani in Saheeh al-Tirmidhi.
This hadeeth indicates that one should avoid going to extremes in rinsing the nose when fasting, so that the water will not inadvertently enter the fasting person’s body.
If the fasting person rinses his mouth or nose and some of the water goes down into his throat without him intending it to, that does not break the fast, because Allaah says (interpretation of the meaning):
“And there is no sin on you concerning that in which you made a mistake, except in regard to what your hearts deliberately intend”
This person did not deliberately intend to do that which invalidates the fast, so his fast is still valid.
See al-Sharh al-Mumti’ (6/240, 246).
See also question no. 40698.
And Allaah knows best.