Is it permissible for one who has called the Adhan within the time limit (Waqt) to leave the mosque?
Chapter on Adhan
Al-Mughni
Book of Prayer
Primary text
It is disliked (Makruh) for one who has called the Adhan within the prescribed time to leave the mosque, unless there is a specific need, after which he should return. This restriction exists because the prayer might be established and he would be needed, thus not found. Ahmad states that if one calls the Adhan while in a state of ritual purity (Wudu') at the time of prayer, he should not leave the mosque until he prays, unless he has a necessity.
Supporting text
If the Adhan is called before the prescribed time, such as before the Fajr prayer time, leaving is permissible because his presence for the immediate prayer establishment is not required.