Intention (niyyah) in Islamic practice is a resolution formed in one's heart. For fasting during Ramadan, it is obligatory to establish this intention internally before the Fajr (dawn) prayer. This requirement is based on a hadith of Prophet Muhammad (peace be upon him), who said: ""Whoever does not make his intention before Fajr, there will be no fasting for him."" (Narrated by Abu Dawud)

It's important to note that the intention for fasting is to be made in the heart; there is no requirement to verbalise it. Your fast is considered valid if you have mentally resolved to fast the next day, or if you have generally intended to fast for the entire month of Ramadan.

This understanding of intention aligns with the Islamic principle that actions are judged by intentions, as stated in another well-known hadith. The emphasis on internal resolution rather than verbal declaration underscores the importance of sincerity and mindfulness in acts of worship.

For practical purposes, this means that as long as you go to sleep with the awareness that you will be fasting the next day, or you have a general intention to fast throughout Ramadan, your fast is valid. There's no need for a specific verbal declaration each night, though some may find it helpful as a personal reminder or reinforcement of their intention.