Nahj al-Balāgha

Hadith 2011
repentance
فَعِنْدَ ذلِكَ تَقُولُ: أَسْتَغْفِرُاللهَ.
On such an occasion you may say “astaghfiru’llāh”.
Source: al-Sharīf al-Raḍī