It was narrated from 'Aishah that she said:
"O Messenger of Allah, what do you think I should say in my supplication, if I come upon Laylatul-Qadr?" He said: "Say: 'Allahumma innaka 'afuwwun tuhibbul-'afwa, fa'fu 'anni (O Allah, You are Forgiving and love forgiveness, so forgive me).'"

حَدَّثَنَا عَلِيُّ بْنُ مُحَمَّدٍ، حَدَّثَنَا وَكِيعٌ، عَنْ كَهْمَسِ بْنِ الْحَسَنِ، عَنْ عَبْدِ اللَّهِ بْنِ بُرَيْدَةَ، عَنْ عَائِشَةَ، أَنَّهَا قَالَتْ يَا رَسُولَ اللَّهِ أَرَأَيْتَ إِنْ وَافَقْتُ لَيْلَةَ الْقَدْرِ مَا أَدْعُو قَالَ " تَقُولِينَ اللَّهُمَّ إِنَّكَ عَفُوٌّ تُحِبُّ الْعَفْوَ فَاعْفُ عَنِّي " .