Wa `alaykum as-Salamu wa Rahmatullahi wa Barakatuh.

 

The Qur’an provided exceptions for those who are ill, “[Fasting for] a limited number of days. So whoever among you is ill or on a journey [during them] – then an equal number of days [are to be made up]. And upon those who are able [to fast, but with hardship] – a ransom [as substitute] of feeding a poor person [each day]. And whoever volunteers excess – it is better for him. But to fast is best for you, if you only knew.” (Al-Baqarah 2:184)

Scholars defined the type of an illness that allows a Muslim to break his fast. Fasting should cause an increase in an existing illness or a delay of recovery or brings up a great difficulty for a fasting person.

If dehydration due to fasting will badly affect your health or increase your illness, you can break your fast and make up those days you have missed later.
If you are not able to fast in other days, you can feed a needy person for each day you are going to miss.

Allah Almighty knows best.