للَّهمَّ إِنِّي أَسْأَلُكَ بِرَحْمَتِكَ الَّتِي وَسِعَتْ كُلَّ شَيء،ٍ أَنْ تَغْفِرَ لِي
‘Abdullah Ibn Amr Ibn Al-Aas (ra) related that the Messenger of Allah (saw) said:
‘Indeed the fasting person has at the time of breaking fast, a supplication which is not rejected’. Ibn Abee Mulaykah said: ‘I Heard Abdullah Ibn Umar say when he broke his fast:
‘O Allah, I ask You by Your mercy which envelopes all things, that You forgive me.’
Source: Hisnul Muslim