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

Allâhumma innî as’aluka bi-rahmatika l-lâti wasi^cat kulla shay’in an taghfira lî.

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

Source: Ibn Majah 1/557