ENOfficial
Usama b. Zaid reported God’s Messenger as saying, “If one is done a kindness and expresses to his benefactor a desire that God may give him a good reward, he has fully expressed his commendation.” Tirmidhi transmitted it.
Source: sunnah.com
Other translations
Arabic
وَعَنْ أُسَامَةَ بْنِ زَيْدٍ قَالَ: قَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ: " مَنْ صُنِعَ إِلَيْهِ مَعْرُوفٌ فَقَالَ لِفَاعِلِهِ: جَزَاكَ اللَّهُ خَيْرًا فَقَدْ أَبْلَغَ فِي الثَّنَاءِ ". رَوَاهُ التِّرْمِذِيّ
sunnah.com