Jazakallahu Khair

Usama ibne Zaid RA narrates that Rasulullah SAW said: If the recipient of a kindness says to his benefactor Jazakallhu Khair (may Allah give you a better reward), indeed he has fully praised and been thankful. (Tirmidhi)

Note: To pray in these words express that I am unable to give you a better reward for this. Thus, these words carry praise for the benefactors. (Maariful Hadith)