Narrated Usama bin Zaid (RA):

Allah's Messenger (ﷺ) said. "If one is done a kindness and [expresses his gratitude] to his benefactor by saying 'May Allah give you a good reward', he has fully expressed his appreciation." [at-Tirmidhi reported it, and Ibn Hibban graded it Sahih (authentic)].

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]‏ .‏