Assalamualaikum warahmatullah wabarakathu,
In the name of Allah the most kind the most merciful.
Glory be to Him who does not treat the citizens of His kingdom unjustly.
Glory be to Him who does not let the torment and pain come suddenly upon mankind.
Glory be to the compassionate the merciful.
O Allah, fill my heart with true enlightenment, wisdom, understanding and knowledge.
Surely you are able to do all thing,
I beseech You, O He who reminds about doing good, does good and commands to do good. Send blessings on Muhammed and the family of Muhammed and let me remember what shaytan makes me forget.
Nabi Muhammad SAW said, "If good is done to someone and then they say "Jazzak ALLAHU Khairan" to the one who did the good, they have indeed praised them well." Hadith narrated by Imam Tirmidhi
There are a few common ways to say jazzakallah, among them are:-
a) jazzakallahu khairan – this is to a man
b) jazzakillahu khairan – this is to a woman
c) jazzakumullahu khairan – to many
Meaning of ‘Jazzakallah’ is; 'may Allah reward you', while meaning of ‘khairan’ is ‘(in) goodness’. The full meaning is 'May Allah reward you [in] goodness'
Answering the above is usually:
a) Wa iya ka – by a man
b) Wa iya ki – by a woman
c) Wa iya kum – to many
Meaning of ‘Wa Iya Kum’ is; ‘And goodness to you also’.
The most common response to ‘jazzakallah’ is ‘wa iya kum’, but it is best that we practice saying ‘amen, wa iya kum’ which is much better. It has amen for the doa and also you return those good doa to the person.
There is also some other way of reply such as “Wa antum Fa JazzakumuALLAHu Khairan” which mean “and in-fact to you, may Allah reward you [in] goodness also."
May ALLAH SWT shed true light to all.