The Prophet (ﷺ) used to say when he breaks his fast:

ذَهَبَ الظَّمَاءُ وَابْتَلَّتِ الْعُرُوْقُ وَثَبَتَ الْأَجْرُ إِنْ شَاءَ اللَّهُ

Dhahabadh-dhama'u wabtallatil-'urooqu, wa thabatal-'ajru 'inshaa'Allah

The thirst is gone, the veins are moistened and the reward is confirmed, if Allah wills. Note that, the widely practiced hadith 'Allahumma laka samtu wa 'ala rizqika aftartu' but, the hadith is da'if (weak). [2]

Reference: [1] Hasan (Albani). Abu Dawud: 2357 [2] Weak (Albany). Abu Dawud: 2358