What is the ruling on the dawn (Fajr) adhan (call to prayer) made by a person who was in a state of ritual impurity (junub) and forgot about this impurity until he finished the adhan, then he performed ghusl (ritual bath) and returned to lead the prayer while pure, especially given the prohibition of entering the mosque while in a state of ritual impurity?

Purity for the Adhan (definition) is recommended, not obligatory. Thus, the Adhan is valid from one who is in a state of minor or major ritual impurity. However, it is preferable for the Mu'adhdhin to be pure from both states of impurity. It is forbidden for one who is in a state of major ritual impurity (junub) to remain in the mosque unless he performs ablution (wudu). If he performs wudu, then it is permissible for him to stay and sleep in it. If the Mu'adhdhin enters the mosque forgetting his state of major ritual impurity, there is no blame upon him.