Reference: http://mufti.af.org.sa/ar/content/الدعاء-المستحب-عند-الفطر
Question: What is the recommended supplication [to utter] at the time of breaking the fast?
Response: It has been narrated that when the Prophet ﷺ would break his fast, he would say:
«The thirst is gone, the veins are moistened, and the reward is established – if Allaah wills.» – Abu Daawood, 2357.
It has [also] been narrated [that He ﷺ would say]:
«O Allaah, for you I fasted, and by your provision, I [now] break the fast.» – Abu Daawood, 2358.
…that said however, the time of breaking the fast is a time when the supplication [to Allaah] is accepted; so the Muslim should seek out those beneficial and encompassing supplications – and supplicate with them seeking what he desires, with hope in Allaah [that He will accept the supplication].