اللّهُـمَّ إِنَّـي أَسْـأَلُـكَ بِرَحْمَـتِكَ الّتي وَسِـعَت كُلَّ شيء، أَنْ تَغْـفِرَ لي
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.
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.