Hisnul Muslim 177

Upon breaking fast

اللَّهُمَّ إِنِّي أَسْأَلُكَ بِرَحْمَتِكَ الَّتِي وَسِعَتْ كُلِّ شَيْءٍ، أَنْ تَغْفِرَ لِي

Allaahumma 'innee 'as'aluka birahmatikal-latee wasi'at kulla shay'in 'an taghfira lee.

O Allah, I ask You by Your mercy, which encompasses all things, that You forgive me.