Dua before salam — forgiveness
اللّهُـمَّ إِنِّـي ظَلَـمْتُ نَفْسـي ظُلْمـاً كَثـيراً...
اللّهُـمَّ إِنِّـي ظَلَـمْتُ نَفْسـي ظُلْمـاً كَثـيراً وَلا يَغْـفِرُ الذُّنـوبَ إِلاّ أَنْت، فَاغْـفِر لي مَغْـفِرَةً مِنْ عِنْـدِك وَارْحَمْـني، إِنَّكَ أَنْتَ الغَـفورُ الرَّحـيم
Allahumma innee thalamtu nafsee thulman katheeran wala yaghfiruth-thunooba illa ant, faghfir lee maghfiratan min AAindik warhamnee, innaka antal-Ghafoorur-Raheem
Translation
O Allah, I have indeed oppressed my soul excessively and none can forgive sin except You, so forgive me a forgiveness from Yourself and have mercy upon me. Surely, You are The Most-Forgiving, The Most-Merciful.
When to recite
Recited after the last tashahhud and before salam in prayer.
Source / Reference
Al-Bukhari 8/168, Muslim 4/2078