Recite this Dua in the last 10 days of Ramadan especially on the odd numbered nights 21,23,25,27.29 because the Night of Decree (Lailatul Qadr) is one of these odd numbered nights:
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'Aishah (May Allah be pleased with her) reported:
I asked: "O Messenger of Allah! If I realize Lailat-ul-Qadr (Night of Decree), what should I supplicate in it?" He (ﷺ) replied, "You should supplicate: Allahumma innaka 'afuwwun, tuhibbul-'afwa, fa'fu 'anni (O Allah, You are Most Forgiving, and You love forgiveness; so forgive me)."
[At-Tirmidhi]
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Sunnah.com reference
: Book 9, Hadith 205
Arabic/English book reference
: Book 9, Hadith 1195
Narrated Abu Huraira:
Allah's Messenger (ﷺ) said, "Whoever establishes the prayers on the night of Qadr out of sincere faith and hoping to attain Allah's rewards (not to show off) then all his past sins will be forgiven."
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Reference
: Sahih al-Bukhari 35
In-book reference
: Book 2, Hadith 28
USC-MSA web (English) reference
: Vol. 1, Book 2, Hadith 35