Dua to Start Your Fast

No masnun dua is reported for the start of a fast. What matters is to form the intention to fast on the night before the fast begins. Intention means the conscious aim held in the mind. So the believer's intention to fast is made before fasting, either before going to bed at night or at sahri. Saying "Nawaituan", or voicing it in any other way, is not Sunnah.