The Prophet (PBUH) had a certain supplication for every moment of his life, and he had a special supplication when coming to break his fast which has been related by Abu-Dawud, ranked Hasan by Al-Albani, that Ibn Umar  said that the Prophet (PBUH) used to say when breaking his fast: ( Thirst has gone, the arteries are moist, and the reward is sure, if Allah wills.)

This supplication indicates the joy of the Prophet for completing his fast, and for attaining reward.

http://300035.dotdothktrade.tech/en/artical/28792/24-sunnah-supplications-relating-breaking-fast