Authentic Supplication Number 167.

The Messenger of Allah  said: “whoever has good done to him, and says to the doer Jazaka ‘allahu khayran (186), then he has excelled in praise.”

(186) May Allah reward you well.

‘Al-`Adawee said it was comely, and that it was reported by At-Tirmithi, page 185 of volume 6 of Tuhfah; and Ibn as-Sunni in ‘Al-Yawm wal-Laylah, #276. Al-Albani said it was sound in his Saheeh Al-Jami`i As-Sagheeri wa Ziyadatihi #6368.