Bathing, Using Mouthwash, and Istinsyaq during Fasting…
From Umar RA: he said: One day my passion arouse then I kissed (my wife), and I was fasting. So I went to the Prophet SAW. Then he
said: “Today I have done a big problem, I’ve kissed (my wife) and I’m fasting.” Then the Prophet Muhammad SAW said, “What do you think when you rinse with water (mouthwash) when you are fasting?” “That’s okay,” I replied. “So what else (do you want to ask)?” Said the Prophet as well (hadith Narrated by Abu Dawood, Ahmad, and An-Nasa’i; Ibn Huzaimah, Ibn Hibban and Al Hakim declared it is an authentic hadith)
With the statement of the Prophet SAW means it does not invalidate to rinse the mouth during fasting.
And from Abu Bakr bin Abdurrahman, a friend of the Prophet, he said: “I saw the Prophet pour water over his head to lessen the heat when he was fasting (Hadith Narrated by Abu Dawood, An-Nasa’i and Ahmad ; its sanad people are people of legitimate hadeeth).
This Hadith shows that for the fasting person it is allowed to pour water over his head to reduce heat. Also to flush part or all of the body is allowed according to scholars. They do not distinguish between shower wajib (obligatory), sunnah or mubah (permissible). Only madhab Hanafi considers it makruh to shower while one is fasting.
So if it is simply to reduce the heat, bath is permissible, as well as gargling and inhaling water into the nose (istinsyaq), but it is makruh if too much rinse and istinsyaq .
Source: Fiqhul Mar’ah Al-Muslimah by Ibrahim Muhammad Al-Jamal