The Hadeth " Some Verdicts Pertaining to Supplications"
عن أسامة بن زيد رضي الله عنه قال قال رسول الله صلى الله عليه و سلم : [ من صنع إليه معروف فقال لفاعله : جزاك الله خيرا فقد أبلغ في الثناء ] رواه الترمذي وقال حديث حسن صحيح1496. Usamah bin Zaid (May Allah be pleased with them) reported that: The Messenger of Allah (PBUH) said he who is favored by another and says to his benefactor : 'jazaka allaho khairan' (may allah raward you well), indeed praised (the benefactor) satisfactorily.[(At-Tirmidhi.'
Commentary: if someone is unable to return an other one's kindness with kindness, he should say'' jazaka- Allaho khairan, 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 peace and blessings be upon him said that it was the best praise for a benefactor.