The below dua is something that we have been thought as the niyah for fasting since our childhood.
*Allahumma Asumu Gadallaka Fagfirli
MA Qaddamtu Wa Ma Akhartu*
When i checked for its authenticity, there isn't any dalil for this dua from the books of Sunnah. Not even a weak hadith is available. On top of it, this dua itself is wrong when u check its translation.
Translation: “O ALLAH! I shall keep fast
*tomorrow* .” (Therefore forgive all my past & future sins).
In Islam the day begins at Mughrib and ends at Mughrib. Then the dua for suhoor would have been "I shall keep my fast *Today* " but the one who has fabricated this dua would have done a mistake and have made it tomorrow.
With regard to sahoor, there is no specific du’aa’ to be said at this time. What is prescribed is to say Bismillah at the beginning and to praise Allah.
With regard to the intention, it is not prescribed to utter it out loud. Shaykh al-Islam Ibn Taymiyah said: “Whoever thinks
in his heart that he will fast tomorrow has formed the intention.”
Abstain from innovations, for every kind of innovation is a bid'ah, and every bid'ah is misguidance and all misguidance
leads to hell fire." [Sunan Abu Dawud, Kitab-us- Sunnah, Book 40 Hadith number 4590].
May Allah save us from these kind of Bidah and guide us to the right path.