When beginning the fast – Suhur (sehri)
بِصَوْمِ غَدٍ نَوَيْتُ مِنْ شَهْرِ رَمَضَانَ
I intend to keep the fast for tomorrow in the month of Ramadan.
Bi-sawmi ghadin nawaytu min shahri ramadan
There is no special Du’aa to begin the fast. You can make the Niyyat in your own language e.g. O Allah I am fasting tomorrow for you. So make it easy for me and accept it from me. The du’aa provided here is an example.
Niyyat means the intention of heart and one eats sehri with the intention of fasting. Then eating sehri is tantamount to intention. There is no need of intention in words separately but if one reads the said du’aa in sehri then it is lawful and better. [Darul Ifta, Darul Uloom Deoband, India]