"What should I say during Laylatul Qadr?"

`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:

اللَّهُمَّ إِنَّكَ عَفُوٌّ تُحِبُّ الْعَفْوَ فَاعْفُ عَنِّي

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

(O Allah! You are forgiving, and you love forgiveness. So forgive me)

[Sunan Ibn Majah]

And in another narration:

اللَّهُمَّ إِنَّكَ عُفُوٌّ كَرِيمٌ تُحِبُّ الْعَفْوَ فَاعْفُ عَنِّي ‏
"Allahumma innaka 'afuwwun kareem tuhibbul 'afwa fa'fu 'annee"
(O Allah! You are the one who forgives greatly, and loves to forgive. So forgive me)

[Sunan Tirmidhi]