The Prophet ﷺ said, "If one of you hears the call to prayer while the vessel is still in his hand, he should not put it down until he has finished what he needs from it." The caller used to give the call as soon as Fajr broke.
حدثنا روح حدثنا حماد عن عمار بن أبي عمار عن أبي هريرة عن النبي ﷺ مثله وزاد فيه وكان المؤذن يؤذن إذا بزغ الفجر