The dua for breaking fast is: 'Dhahaba al-dhama'u wabtallatil-'uruqu wa thabatal-ajru in sha Allah' — The thirst has gone, the veins have been moistened, and the reward is confirmed if Allah wills. This is recited at the moment of breaking the fast at Maghrib time.
ذَهَبَ الظَّمَأُ وَابْتَلَّتِ الْعُرُوقُ وَثَبَتَ الْأَجْرُ إِنْ شَاءَ اللَّهُ
Dhahaba al-dhama'u wabtallatil-'uruqu wa thabatal-ajru in sha Allah.
Meaning: The thirst has gone, the veins have been moistened, and the reward is confirmed, if Allah wills.
Source: Sunan Abu Dawud #2357The moment of breaking the fast is one of the times when dua is most accepted. The Prophet (peace be upon him) said: 'Three supplications are not rejected: the supplication of a fasting person at the time of breaking the fast, the supplication of a just ruler, and the supplication of the oppressed.' (Sunan al-Tirmidhi 3598)
ذَهَبَ الظَّمَأُ وَابْتَلَّتِ الْعُرُوقُ وَثَبَتَ الْأَجْرُ إِنْ شَاءَ اللَّهُ
When the Prophet (peace be upon him) broke his fast, he would say: 'Dhahaba al-dhama'u wabtallatil-'uruqu wa thabatal-ajru in sha Allah.'