وَبِصَوْمِ غَدٍ نَّوَيْتُ مِنْ شَهْرِ رَمَضَانَ

Transliteration: Wa bisawmi ghadinn nawaiytu min shahri Ramadan

Translation: "I intend to keep the fast for tomorrow in the month of Ramadan."

Du'a for starting a fast