Here s a beautiful dua to be said before beginning the fast at suhoor time. Sahl ibn Sa’d reported that the Messenger of Allah (saw): “There is a gate to Paradise that is called ar-Rayyan. On the Day of Resurrection it will say: ‘Where are those who fasted?’ When the last [one] has passed through the gate, it will be locked.” [Imam Bukhari and Imam Muslim].

وَبِصَوْمِ غَدٍ نَّوَيْتَ مِنْ شَهْرِ رَمَضَانَ

 Wa bisawmi ghadinn nawaiytu min shahri ramadan 

I intend to keep the fast for tomorrow in the month of Ramadan