He doesn’t have to do that for one or both ears. It is recommended in some schools of thought because is makes the sound louder.

Why Muadzin must closed 1 of his ears while giving adhan .

Prayer | February 11, 2014 1:12 pm
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Can we make the intention of doing certain good deeds lyk offering nawafil salah or fasting for some days if our some important work is done? Did the prophet or his companions do it.