اللْهُمَّ إِنَّكَ عَفُوٌّ تُحِبُّ الْعَفْوَ فَاعْفُ عَنِّي

Ummul Mu'mineen, Aisha, may Allah be pleased with her, said:

I asked the Messenger of Allah: 'O Messenger of Allah, if I know what night is the night of Qadr, what should I say during it?'

He (The Prophet PBUH) said:

'Say: O Allah, You are pardoning and You love to pardon, so pardon me.'

(Ahmad, Ibn Majah, and Tirmidhi).

The transliteration of this Dua is

"Allahumma innaka `afuwwun tuhibbul `afwa fa`fu `annee"

 
اللْهُمَّ إِنَّكَ عَفُوٌّ
O Allah, You are pardoning

تُحِبُّ الْعَفْوَ
and You love to pardon

فَاعْفُ عَنِّي
so pardon me

(Taken from here)