Reference : Hisn al-Muslim 177

اللّهُـمَّ إِنَّـي أَسْـأَلُـكَ بِرَحْمَـتِكَ الّتي وَسِـعَت كُلَّ شيء، 
أَنْ تَغْـفِرَ لي
Allāhumma innī as'aluka 
bi raḥmatika ‘l-latī wasi`at kulla shay' 
an taghfira lī.


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

Reference:
Ibn Majah 1/557 from a supplication of Abdullah bin 'Amr. Al-Hafidh graded 
it as good in his checking of An-Nawawi's Kitabul-'Athkdr. See Sharhul- 
Athkar 4/342.

Chapter: Upon breaking fast