Is it permissible to perform supererogatory prayers (nafilah) with the intention of praising Allah (Alhamdulillah) for an event, or with the intention of seeking forgiveness (Istighfar)?

It is prescribed for one who has sinned to perform ablution and pray two rak'ahs, based on the Prophet's (peace be upon him) saying: "No servant commits a sin, then purifies himself well, then stands and prays two rak'ahs, then seeks Allah's forgiveness, but Allah forgives him." As for one who sees something that pleases him, it is better to prostrate in gratitude to Allah, and there is no harm in praying two rak'ahs with the intention of showing gratitude, as prayer is among the acts of obedience with which a servant responds to Allah's blessings. The term "prayer of gratitude" is used metaphorically because it is an act of obedience among acts of obedience, and it is not a specific prayer called the prayer of gratitude. The Prophet (peace be upon him) indeed performed a ghusl and prayed eight rak'ahs as a form of gratitude to Allah Almighty when Mecca was conquered.