اللَّهُمَّ اغْفِرْ لِي ذَنْبِي كُلَّهُ وَتُبْ عَلَيَّ إِنَّكَ أَنْتَ التَّوَّابُ الرَّحِيمُ
Transliteration
Allahummaghfir li dhanbi kullahu wa tub 'alayya innaka antat-tawwabur-Rahim
English meaning
O Allah, forgive all my sins and accept my repentance. Verily, You are the Accepter of repentance, the Most Merciful.
When is it recited?
Recited with remorse after committing a sin.
Source
Muslim, Salat 216, 483; Tabarani