Aisha Siddiqah bint al-Siddiq (RA), "What if I knew which night Laylatul Qadr was, then what should I say in it?"

He (SAWS) said, "Say

اللهم إنك عفو تحب العفو فاعفُ عني

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

O Allah! You are The One Who forgives a lot and You love to forgive so forgive me.

(Reported by Tirmidhi and Ibn Majah)