:sl:
This link has some du'as that are commonly recited after prayer:
http://www.islamawareness.net/Dua/Fortress/025.html
Regarding the specific one you mentioned, there is a hadeeth about it which says:
`A'ishah (radhiallahu `anha) reported that she asked Allah's Messenger (sallallahu `alayhi wa sallam), "O Messenger of Allah! If I knew which night is Laylat ul-Qadr, what should I say during it?" And he instructed her to say:
“Allaahumma innaka ‘afuwwun tuhibbul-‘afwa fa’fu 'annee”
“O, Allah! You are the Forgiving, You love to forgive. Forgive me!”
[Recorded by Ahmad, Ibn Majah, and at-Tirmithi. Verified to be authentic by Al-Albani]
And Allaah (Swt) knows best.