Importance of Jazak-Allah khairan, Dua for benefactor,Islamic Dua for thanking someone, Dua for the one who helped you, supported you and done a favour on you.

 Usamah bin Zaid (May Allah be pleased with them) reported: The Messenger of Allah (sallallaahu ’alayhi wa sallam) 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.''
[At-Tirmidhi].
Commentary:  If one is unable to return someone's kindness with kindness, he should say, "Jazak-Allahu khairan (may Allah reward you well).'' This means: "I am unable to reciprocate your kindness. May Allah grant you the best reward for it.'' Obviously there is no match for the reward that one gets from Allah. For this reason the Prophet (sallallaahu ’alayhi wa sallam) said that it was the best of the praise for a benefactor.