Jami` at-Tirmidhi · No. 2035Chapter 27
" مَنْ صُنِعَ إِلَيْهِ مَعْرُوفٌ فَقَالَ لِفَاعِلِهِ جَزَاكَ اللَّهُ خَيْرًا فَقَدْ أَبْلَغَ فِي الثَّنَاءِ "
"Whoever some good was done to him, and he says: 'May Allah reward you in goodness' then he has done the most that he can of praise."