Dua 59 Every believer should keep reciting this prayer:
(Surah Al-Mu'minun (23), Verse 109)

رَبَّنَآ ءَامَنَّا فَٱغۡفِرۡ لَنَا وَٱرۡحَمۡنَا وَأَنتَ خَيۡرُ ٱلرَّـٰحِمِينَ

Rabbanaaa aamannaa faghfir lanaa warhamnaa wa Anta khairur raahimeen.

Our Lord! We have believed. So forgive us, and have mercy on us, and You are the best of the merciful.

(Surah Al-Mu'minun (23), Verse 109)

On the Day of Judgment, Allah the Almighty will remind the polytheists, hypocrites, and disbelievers: "When you (all three groups) used to mock the believers in the world, My faithful servants would pray with this supplication. Now, I will surely grant them My refuge, forgive their sins, and shower them with My mercy."
Hadith mentions that noble companions like Hazrat Bilal (R.A.), Hazrat Ammar (R.A.), Hazrat Suhaib (R.A.), and the devoted lovers of Imam Ali (A.S.) used to recite this prayer.