Al-Kāfi - The Book of Fasting

Chapter: Rinsing One’s Mouth and Nostrils while Fasting ِنْ أَصْحَابِنَا عَنْ سَهْلِ بْنِ زِيَادٍ عَنِ الرَّيَّانِ بْنِ الصَّلْتِ عَنْ يُونُسَ قَالَ الصَّائِ٠َضَانَ يَسْتَاكُ ٠َتَى شَاءَ وَإِنْ تَ٠َضَ فِي وَقْتِ فَرِيضَتِهِ فَدَخَلَ الْ٠َاءُ حَلْقَهُ فَلَيْسَ عَلَيْهِ شَيْ‏ءٌ وَقَدْ تَ٠َضَ فِي غَيْرِ وَقْتِ فَرِيضَةٍ فَدَخَلَ الْ٠َاءُ حَلْقَهُ فَعَلَيْهِ الإِعَادَةُ وَالأَفْضَلُ لِلصَّائِ٠4.

A number of our people have narrated from Sahl ibn Ziyad from al-Rayyan ibn al-Salt from Yunus who said: “One who is fasting in the month of Ramadan can brush his teeth whenever he wants; but if one rinses his mouth during the time of obligatory prayers, and water enters his throat, there is nothing upon him, and he can complete his fast. And if one rinses his mouth in a time other than the time of obligatory prayers, and water enters his throat, one must repeat it (fast again).

And it is better for the one fasting not to rinse his mouth.’” شهور - Allamah Baqir al-Majlisi (Mirʾāt al-ʿUqūl fī Sharḥ Akhbār Āl al-Rasūl (16/285))