Saying “Jazak Allahu khayran” (“May Allah reward you generously”) is established from the sunnah of the Beloved Messenger of Allah (pbuh).
The Messenger of Allah (pbuh) said, “If good is done to someone and then they say “Jazak Allahu khayran” to the one who did the good, they have indeed praised them well.” [Tirmidhi]
What should one reply if someone says Jazak Allahu khayran to him?
There is a hadeeth in Saheeh Ibn Hibbaan in which the Sahaabi said Jazak Allahu Khyran to the Messenger (pbuh) and in reply he said
And you too, May Allah reward to with Khayr.
The Hadeeth is narrated by Usayd Ibn Hudair
wa iyyaka wa jazaka
â€œAnd you as wellâ€ (wa iyyaka) or â€œAnd may He reward you.â€ (wa jazaka)
Or we can also reply using ,,,,
“wa anta fajazakallahu khaira“.
When two or more companions said to the holy prophet (sallallahu alyhi wa sallam) jazak Allahu khair – the holy prophet replied
“wa antum fajazakumullahu khaira“.
The word wa-iyyaakum has the same meaning and is the shorter version of wa-antum-fajazakumullahu-khair. Not a big deal though, but it is always better to use the words that are transmitted in sunnah than words that we invent.
The correct way of saying May Allah reward you with good in different grammatical forms, i.e. to a man, woman, group etc.:
To a male: Jazak Allahu khayran
To a female: Jazaki-llahu khayran
To two people (whether male/female): Jazakuma Allahu khayran
To three men or more: Jazakum Allahu khayran
To three women or more: Jazakumunna-llaahu khayran