مَنْ صُنِعَ إِلَيْهِ مَعْرُوفٌ، فَقَالَ لِفَاعِلِهِ: جَزَاكَ اللَّهُ خَيْرًا، فَقَدْ أَبْلَغَ فِي الثَّنَاءِ
Translation: Whoever receives a kindness and says to the doer, “May Allah reward you with good,” has expressed excellent gratitude and praise.
Source: الترمذي
صُنِعَ is a passive past-tense verb: “a kindness was done,” leaving the doer unnamed initially. لِفَاعِلِهِ points to the person who performed the kindness. جَزَاكَ اللَّهُ خَيْرًا is a supplication: “May Allah reward you with good.” Finally, فَقَدْ أَبْلَغَ فِي الثَّنَاءِ describes that response as an excellent expression of thanks.
Gratitude strengthens relationships. A simple “thank you” is courteous; the report adds a prayer for the one who helped. In this way, appreciation is joined to seeking goodness for another person.