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.
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.