مَنْ فَطَّرَ صَائِمًا كَانَ لَهُ مِثْلُ أَجْرِهِ غَيْرَ أَنَّهُ لَا يَنْقُصُ مِنْ أَجْرِ الصَّائِمِ شَيْئًا
Man fattara sa'iman kana lahu mithlu ajrihi ghayra annahu la yanqusu min ajris-sa'imi shay'a
Whoever provides iftar to a fasting person will have the same reward as him, without diminishing anything from the fasting person's reward.
While this is a hadith rather than a du'a, it is a powerful reminder during Ramadan. Even providing a single date or glass of water to someone breaking their fast earns you the full reward of their entire day of fasting.
Related Du'as
اللَّهُمَّ أَهِلَّهُ عَلَيْنَا بِالْأَمْنِ وَالْإِيمَانِ وَالسَّلَامَةِ وَالْإِسْلَامِ رَبِّي وَرَبُّكَ اللَّهُ
O Allah, let this moon rise upon us with security, faith, safety, and Islam. My Lord and your Lord is Allah.
Hasanاللَّهُ أَكْبَرُ اللَّهُ أَكْبَرُ لَا إِلَٰهَ إِلَّا اللَّهُ وَاللَّهُ أَكْبَرُ اللَّهُ أَكْبَرُ وَلِلَّهِ الْحَمْدُ
Allah is the Greatest, Allah is the Greatest. There is no deity except Allah. And Allah is the Greatest, Allah is the Greatest, and to Allah belongs all praise.
Sahihذَهَبَ الظَّمَأُ وَابْتَلَّتِ الْعُرُوقُ وَثَبَتَ الْأَجْرُ إِنْ شَاءَ اللَّهُ
The thirst is gone, the veins are moistened, and the reward is confirmed, if Allah wills.
Hasanاللَّهُمَّ بَلِّغْنَا لَيْلَةَ الْقَدْرِ وَاجْعَلْنَا فِيهَا مِنَ الْمَقْبُولِينَ وَلَا تَحْرِمْنَا خَيْرَهَا وَاكْتُبْ لَنَا فِيهَا الْعِتْقَ مِنَ النَّارِ
O Allah, let us reach Laylatul Qadr, make us among the accepted in it, do not deprive us of its goodness, and decree for us in it freedom from the Fire.
Athar