It is part of the Sunnah. The Prophet salla Allahu alaihi wa sallam used to have two Moa’thins and one of them (Bilal) would call for the Athan before the time for Fajr is due so that those who are asleep would wake up and those who are intending to fast would be able to eat something.

In Sudan, any person come to Masjid in morning before Fajar, they do Azan in laud speaker and call this Azan Tanbih, Jawab with this is Azan is also necessary or not as this is not for Salat. Also this Azan interrupt the sleep of some people like children and sick people, is this Azan Sunnah or not?

Rulings on mosques | March 13, 2014 3:29 pm
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What are the requirements for a place (say like a room) to be called a mosque? If I always manage to get 4-5 people and form a congregation in the room of my house, but I don’t pronounce Adhan,would I be waived of the obligation of going to the mosque that is 2 to 3 minutes away from my house? Would I get the 27 times manifold reward?