Sunnah Way of Thanking Others: Jazakallah or Jazakallah Khair?

In the name of Allah

Assalamu aleikum wa rahmatullahi wa barakatuhu

Understanding Grammar Behind The word Jazakallah
Jazaak comes from the root word “jazaa” which according to the popular Arabic-English dictionary, Al-Mawrid, has two meanings that are completely opposite to each other!
Thus jazaa can either mean reward OR punishment.
So :"JazakAllah" can may either mean "may Allah reward you" or punish you, while
"JazakAllah Khair" means ... "may Allah reward you with the Best / Good",
So the correct way is to say Jazakallah khair & not just jazakAllah. Though someone might say that the intention by saying jazakallah is the same like jazakallahu khayran, if its so, then why not use the complete wordings as used by prophet(sallallahu aleihi wa sallam) and the sahabas? which is jazakallahu khayran !

Jazak Allahu Khair depending on situation:
Masculine: Jazak Allahu Khair
Feminine: Jazaki Allahu Khair
Plural (m): Jazakum Allahu Khair
Plural (f): Jazakunnna Allahu khair

Substitute for Jazakallah khair is BarakAllah Feek ( "May the blessings of Allah be upon you.")
How to Reply to Those who says Jazakallahu khair to you:

1) Wa Antum fa Jazakumullahu khayran meaning "And you too, May Allah reward to with Khayr"

✰Evidence from Sunnah: Usayd ibn Hadayr (sahabi) says: I said:
O Messenger of Allah ﺟﺰﺍﻙ ﺍﻟﻠﻪ ﺧﻴﺮﺍ [Jazaakallhu Khayran]
The Prophet(sallallahu aleihi wa sallam) said: ﻭَﺃَﻧْﺘُﻢْ ﻓَﺠَﺰَﺍﻛُﻢُ ﺍﻟﻠَّﻪُ ﺧَﻴْﺮًﺍ [Wa Antum Fa Jazakumullahu khayran] And you too, May Allah reward to with Khayr].
[Albaani has said that the Hadeeh is Saheeh in al-Saheeha 3096, al-Ta'leeqaatul hisaan al Saheeh ibn Hibbaan 6231]

2) Wa iyyakum (ﻭﺇﻳﺎﻛﻢ)” meaning “And goodness to you also”
✰This is the one the common message used by people. Muslims can use this phrase sometimes, and abandon it sometimes, but they must not cling to it as if it is an established Sunnah of the Messenger since there is no evidence related to it.
▸ ▹ ► ✰Conclusion✰: It’s a kind suggestion to all members that don’t just be a follower of prophet(sallallahu aleihi wa sallam) by name  but rather by following his sunnah. The correct sunnah is not to say thanks or shukran rather its to say jazakallahu khair or barakAllah feek. and should be replied with Wa Antum fa Jazakumullahu khayran. Let's revive this sunnah in our lives In shaa Allah & motivate others to do the same.

​And Allah knows the best !​