Praying for one who does you a favor

"May Allah reward you with good."

The Prophet ﷺ said: "Whoever has good done to him and says to the one who did it ‹Jazaka-Llahu khayran›, has given him the best of thanks" [at-Tirmidhi 2035].

[at-Tirmidhi 2035; Sahih Sunan at-Tirmidhi 2035]