I would like to know the usage of Jazak Allah Khair.
When is it best used and, is there any restrictions?
What does "Jazak Allah Khair" mean?
When do we use "Jazak Allah Khair"?
Jazak Allah Khair for the answers.
"ka" (ك) here means "to/for you".
So, "jazaka" (جزاك) means "He will reward you"
"Allah" here explicitly refers Allah as the doer (subject of the sentence or فاعل). Now "He" is "Allah".
"Khair" (خير) is the noun meaning, "good".
The full meaning becomes: "Allah will reward you good". This is used as a Du'a by the Muslims and in Arabic it is: جزاك الله خيرا.
It means may Allah give you reward.
In the name of Allah, the Entirely Merciful, the Especially Merciful
Malik ibn A’yin quoted that I asked Imam Sadiq (a.s.):
… a Muslim tells another Muslim: جزاک الله خیرا (Jazak Allah Khaira), what does it mean?
Imam Sadiq (الامام الصادق) (a.s.) said:
(briefly translated): Khair is a creek in the paradise which is originated from Kawthar-Nahr(creek), and at two sides of that “Handmaidens (beautiful (women...) who are related to the paradise)” …
In the meanwhile, it is remarked that it is related to the verse 70 or Surah A’Rahman …
Afterwards, he said:
Whenever a Muslim tells his (her) friend JazakAllah-Khaira, the purpose of “Khair(Khaira)” is the homes (the places (or creeks) in the paradise as it was mentioned before…)…
معانى الاخبار شیخ صدوق ص 1
This expression might has worldly blessings/effects as well as its Okhrawi reward/effect (by surveying more explanations and interpretations, and so forth)
عَنْ أُسَامَةَ بْنِ زَيْدٍ، قَالَ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم "مَنْ صُنِعَ إِلَيْهِ مَعْرُوفٌ فَقَالَ لِفَاعِلِهِ جَزَاكَ اللَّهُ خَيْرًا فَقَدْ أَبْلَغَ فِي الثَّنَاءِ"
Usamah bin Zaid narrated that the Messenger of Allah (Peace be upon him) said:
"Whoever some good was done to him, and he says: jazaakAllahu Khayran ('May Allah reward you in goodness') then he has done the most that he can of praise."
Jami` at-Tirmidhi Vol. 4, Book 25, Hadith 2035
Jazākallāh or Jazāk Allāhu Khayran is a term used as an Islamic expression of gratitude meaning "May Allah reward you [with] goodness.