اللَّهُمَّ إِنِّي أَسْأَلُكَ بِرَحْمَتِكَ الَّتِي وَسِعَتْ كُلَّ شَيْءٍ أَنْ تَغْفِرَ لِي

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

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

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.