Prayer while the adhân is being called

Question

Is it true that one must listen to the adhân first before performing prayer?

Answer

Normally, the person who calls the prayers (the mu’adhdhin) does so on time. He is not allowed to make the call before the actual time. If he calls to early, his doing so is invalid. 

If the mu’adhdhin calls the prayer on time like he is supposed to, the worshipper may pray while hearing the adhân. It is permissible for him to pray since the time for prayer has in fact started.

--islamtoday.net