WA IYYAK

It says in Musannaf Ibn Abi Shaybah (5/322): ‘Umar ibn al-Khattaab (may Allaah be pleased with him) said: If one of you knew what there is in his saying to his brother, ‘Jazaak Allaahu khayran’, you would say it a great deal to one another.

It means “May Allah reward you with a great deal of good” and the good refers to this life and the next.

The Sahaba would say it a lot amongst each other.

Yesterday I learned that the response from the Sunnah after someone says this is: “Wa Anta Fajazakumullahu khairan”

Although Wa Iyyak or Wa anta is okay to say too.