ENOfficial
Narrated Usama bin Zaid (RA): Allah's Messenger (ﷺ) said. "If one is done a kindness and to his benefactor by saying 'May Allah give you a good reward', he has fully expressed his appreciation." .
Source: sunnah.com
Other translations
Arabic
وَعَنْ أُسَامَةَ بْنِ زَيْدٍ رَضِيَ اَللَّهُ عَنْهُمَا قَالَ: قَالَ رَسُولُ اَللَّهِ - صلى الله عليه وسلم -{ "مَنْ صُنِعَ إِلَيْهِ مَعْرُوفٌ, فَقَالَ لِفَاعِلِهِ: جَزَاكِ اَللَّهُ خَيْراً. فَقَدْ أَبْلَغَ فِي اَلثَّنَاءِ" } أَخْرَجَهُ اَلتِّرْمِذِيُّ, وَصَحَّحَهُ اِبْنُ حِبَّانَ 1 .
sunnah.com