As Allaah سبحانه و تعالى says,





"Verily! We have sent it (this Qur'aan) down in the night of Al-Qadr (Decree). And what will make you know what the night of Al-Qadr (Decree) is? The night of Al-Qadr (Decree) is better than a thousand months (i.e. worshipping Allaah in that night is better than worshipping Him a thousand months, (i.e. 83 years and 4 months)). Therein descend the angels and the Rooh [Jibraa'eel (Gabriel)] by Allaah's Permission with all Decrees. Peace! (All that night, there is Peace and Goodness from Allaah to His believing slaves) until the appearance of dawn." (Sooratul-Qadr, 97:1-5)
Ummul Mu'mineen 'Aa'ishah رضي الله عنها narrated that she asked the Prophet صلى الله عليه وسلم: "O Messenger of Allaah! What du'aa' should I make on Laylatul-Qadr (the Night of Decree)?" He صلى الله عليه وسلم answered:
قولي: اللَّهُمَّ إِنَّكَ عَفُوٌّ تُحِبُّ العَفْوَ فَاعْفُ عَنِّي
Allaahumma innaka 'afuwwun tuhibbul-'afwa fa'fu 'annee
"Say, 'O Allaah! You are Forgiving, and love to forgive, so forgive me!'" [2] [3]
Notes:
[1] The Night of Decree is one of the odd nights of the last ten nights of Ramadaan.
[2] Narrated by at-Tirmidhi, Ibn Maajah, and al-Haakim and authenticated in Saheeh al-Jaami' # 4423.
[3] Shaykh Dr. Abu Ammar Yasir Qadhi حفظه الله, Du'a - The Weapon of the Believer, pp. 128-129.