الف سلامة عليك
“alif salams alayk/alayky“
Arabic is funny in the respect that there are sentences/phrases that you are required to use in certain situations; otherwise, whatever you say is just wrong. This is one example of such. When someone says they’re sick, you can either say سلامتك (salamtak/tik – ‘your health’) or الف سلامة عليك and the correct response is الله يسلمك (allah ysalemak/ek).