Allah, The Most Exalted, says:
‘If you are thankful, I will surely increase you (in favor); but if you deny, indeed, My punishment is severe’.” (Qur’an 14:7)
The Noble Messenger of Allah (peace be upon him) is reported to have said:
“He who does not thank people, does not thank Allah”. (Hadith-Tirmidhi)
“If the recipient of a kindness says to his benefactor Jazakallahu Khair (May Allah reward you with goodness), indeed he has fully praised and been thankful.” (Hadith-Tirmidhi)
Indeed one who does not thank those who were kind to him, is not thankful to Allah Ta’ala. We should endeavour to show favour and kindness to those who have benefitted us in whatever way. To pray in these words “Jazakallahu Khair” expresses that I am unable to recompense, so I invoke Allah Subhanahu wa Ta’ala that He may give you a better reward for this. Thus, these words carry praise and benefit for the benefactor.