What should one do if they are assailed by doubt concerning the swallowing of water while performing ablution during a fast?

The fast of someone who doubts having swallowed water during ablution is valid, because the default is the validity of the fast, and certainty is not removed by mere doubt. If he becomes certain that water reached his throat unintentionally, there is no blame upon him, as Allah the Almighty says: (And there is no blame upon you for that in which you have erred, but [blame ensues for] what your hearts have intended). And due to the saying of the Prophet, peace be upon him: "Indeed, Allah has pardoned for my Ummah: error, forgetfulness, and what they are coerced into." However, one must be careful to prevent water from reaching the throat during ablution, due to the saying of the Prophet, peace be upon him: "And go to the extreme in sniffing water up the nose, unless you are fasting."