Reference: Fatawaa Ramadhaan – Volume 2, Page 536, Fatwa No. 510;
Fataawa al-Lajnah ad-Daa.imah lil-Buhooth al-‘Ilmiyyah wal-Iftaa. – Fatwa No.5733
Question: What is the ruling about one who, whilst doing ablution, swallowed some water? Also, one who, whilst fasting, was gargling (not making ablution) and swallowed a drop or two of water unintentionally, or whilst taking a shower or making ablution or cooling [oneself] with water in heat. Must he make up the fast for that day in recompense or should he give in charity to the needy an atonement for these slip-ups knowing that he was overcome and these drops were swallowed without intention. Please give us a fatwa, [and] may Allaah reward you with good.
Response: One who takes a shower or gargles or sniffs some water up his nose and [as a result] water enters his throat without intention, then his fast is not invalidated for that which is narrated from the Prophet ﷺ that he said:
And with Allaah lies all the success, and may Allaah send prayers and salutations upon our Prophet Muhammad ﷺ and his family and his companions.