I asked, "Messenger of Allah, what should I say if I come to know which night is Laylat al-Qadr?"He replied, "Say, 'O Allah, You are pardoning and love to pardon, so pardon me.'"
قلت: يا رسول الله، أرأيت إن علمت ليلة القدر ما أقول فيها؟ قال: قولي: اللهم إنك عفو تحب العفو فاعف عني.
Narrated by Tirmidhi (3513), Ibn Majah (3850), and Ahmad (25384), and Nasai in Amal al-Yawm wal-Layla (872) (73) (874), and Hakim (1/530), and Bayhaqi in Fadail al-Awqat (113), all of them from the hadith of Kahmas bin Hasan, from Abdullah bin Burayda, from Ayesha, who related it. Tirmidhi said, "Hasan sahih," and Nawawi graded it sahih in al-Adhkar. The hadith is discussed in detail in the book on fasting.