جَزاكَ اللهُ خَـيْراً
May Allah reward you with good.
At-Tirmidhi (no. 2035)
Jazakallahu Khairan is an Arabic term used as an Islamic expression of gratitude meaning "May Allâh reward you [with] goodness."
Although the common Arabic word for thanks is shukran (شكراً),
jazak allahu 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 jazak allahu khayran is wa iyyaakum (وإياكم), which means "And to you".
Hadith on saying Jazakallah Khair:
Usamah bin Zaid (May Allah be pleased with them) reported: Rasulullah (Sallallahu alaihi wasallam) said, “He who is favoured by another and says to his benefactor: `Jazak-Allah khairan (may Allah reward you well)’ indeed praised (the benefactor) satisfactorily.”
Riyad As-Salihin – [At-Tirmidhi] Arabic/English book reference: Book 17, Hadith 1496
Hadith on Praising Good:
Usamah bin Zaid narrated that Rasulullah (Sallallahu alaihi wasallam) said: “Whoever some good was done to him, and he says: ‘May Allah reward you in goodness’ then he has done the most that he can of praise.”
Grade: Hasan (Darussalam) Jami At Tirmidhi 2035 Book 27, Hadith 141