In the name of allah the most beneficent the most merciful
Saying “Jazak ALLAHu khayran” (“May ALLAH reward you generously”) is established from the sunnah of the Beloved Messenger of ALLAH (peace and blessings be upon him).
The Messenger of ALLAH (peace and blessings be upon him) 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 JazakALLAHu khayran to him? There is a hadeeth in Saheeh Ibn Hibbaan in which the Sahaabi said JazakALLAHu Khyran to the Messenger (S.A.W) and in reply he said WaAntum Fa JizakAllah.O.Khayran
And you too, May ALLAH reward to with Khayr. The Hadeeth is narrated by Usayd Ibn Hudair (R.A),
Usayd ibn Hadayr says … I said :
O Messenger of ALLAH [JazakALLAHu Khayrn]
The Prophet (S.A.W) said :
And you too, May ALLAH reward you too with Khayr.
Shaykh al-Albaany has said that the Hadeeh is Saheeh.
[al-Saheeha 3096, al-Ta’leeqaatul hisaan al Saheeh ibn Hibbaan 6231]
The actual words that are used in response to JazakALLAH, as narrated in hadith are: “wa anta fajazakallahu khaira”. When two or more companions said to the holy prophet (sallallahu alyhi wa sallam) jazakALLAH 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.