There is no single obligatory phrase, but a supplication reported from the Prophet ﷺ expresses that the thirst is gone, the veins are moistened, and the reward is confirmed if Allah wills. One breaks the fast promptly at sunset, ideally with dates or water.
References
Sunan Abi Dawud 2357
Informational, not a personal fatwa. Consult a qualified scholar for rulings on your situation.