اَللَّهُمَّ إِنِّي أَسْأَلُكَ بِرَحْمَتِكَ الَّتِي وَسِعَتْ كُلَّ 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. Abdullah bin Amr (RA) used to say this dua during Iftar.

Reference: Hasan (Hafiz Ibn Hazar). Takhrijul Adhkar, Sharhul Adhkar: 4/342