JazakAllah Khairan

Jazāk Allāhu Khayran (جَزَاكَ ٱللَّٰهُ خَيْرًا) is a term used as an Islamic expression of gratitude meaning "May Allah reward you with goodness." It asks for Allah SWT to bestow blessings or good to the individual rather than just saying thanks.

Jazakallah is “May Allah reward you”, this itself is incomplete, as it could suggest God reward you with bad or evil.

Muslims should reciprocate kindness, so when one does good to him, the least we can do is say Jazakallah khairan. It's a sign of appreciation and can make the person who did you a favor feel good about themselves.