For the compulsory fast of Ramadan to be accepted, the intention to fast must be made before Fajr each day. The Prophet (peace be upon him) was reported by Hafsah to have said, “Whoever does not intend to fast before Fajr will have no fast.”
As with Salah, there is no verbal formula one says to mark one’s intention. To wake up for the pre-dawn meal is an expression of the intention to fast. Even of one didn’t wake up for suhur, to simply make up one’s mind before sleeping that one will be fasting the next day is sufficient.