This dua is to be said at the time when breaking the fast. ‘Umar said: The Messenger of Allah (peace and blessings of Allah be upon him) used to say when breaking his fast: “Dhahaba al-zama’ wa abtalat al-‘urooq wa thabata al-ajr in sha Allah (Thirst is gone, the veins are moistened and the reward is certain if Allah wills).”

Narrated by Abu Dawood, 2357

The thirst has gone, the veins have been moistened and the reward has been fixed if Allah wills. 

« ذَهَبَ الظَّمَأُ وَابْتَلَّتِ الْعُرُوقُ وَثَبَتَ الأَجْرُ إِنْ شَاءَ اللَّهُ ».