JazakAllah Khayr (Arabic: جزاك اللهُ خيراً) is an Arabic term used as an Islamic expression of gratitude meaning "Allâh will reward you [with] goodness."
Although the common Arabic word for thanks is shukran (شكراً), jazakallahu khayran is often used by Muslims instead, in the belief that one cannot repay a person enough, and that Allâh is able to reward a person the best. Often the response to jazakallahu khayran is wa iyyaakum (وإياكم).