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 بَلِّغْنَا رَمَضَانَ وَأَعِنَّا عَلَى صِيَامِهِ وَقِيَامِهِ وَتَقَبَّلْهُ مِنَّا
O Allah, let us reach Ramadan, help us to fast and pray in it, and accept it from us.
Hasanاللَّهُمَّ تَقَبَّلْ صِيَامِي وَقِيَامِي وَرُكُوعِي وَسُجُودِي
O Allah, accept my fasting, my standing in prayer, my bowing, and my prostration.
Atharاللَّهُمَّ إِنِّي صَائِمٌ وَعَلَى رِزْقِكَ أَفْطَرْتُ فَتَقَبَّلْ مِنِّي إِنَّكَ أَنتَ السَّمِيعُ الْعَلِيمُ
O Allah, I fasted for You and I break my fast with Your provision. So accept it from me. Indeed, You are the All-Hearing, the All-Knowing.
Hasanإِنِّي صَائِمٌ إِنِّي صَائِمٌ
I am fasting, I am fasting. (Say this when provoked while fasting.)
Sahih