Dua upon breaking fast — O Allah forgive me
اللّهُـمَّ إِنِّـي أَسْـأَلُـكَ بِرَحْمَـتِكَ الَّتي وَسِـعَت...
اللّهُـمَّ إِنِّـي أَسْـأَلُـكَ بِرَحْمَـتِكَ الَّتي وَسِـعَت كُلَّ شيء، أَنْ تَغْـفِرَ لي
Allahumma innee as-aluka birahmatikal-latee wasiAAat kulla shay an taghfira lee
Translation
O Allah, I ask You by Your mercy which envelopes all things, that You forgive me.
When to recite
Recited when breaking the fast. The fasting person has at this time a supplication which is not rejected.
Source / Reference
Ibn Majah 1/557