What is the expiation for someone who entered the mosque while in a state of ritual impurity (junub)? May Allah bless you.

There is no harm in a person in a state of janabah passing through the mosque, but they should not reside there, due to the Almighty's saying: ﴿O you who have believed, do not approach prayer while you are intoxicated until you know what you are saying, nor while in a state of janabah, except for those passing through, until you have bathed.﴾ [An-Nisa: 43]. Whoever remains in the mosque while in a state of janabah has committed a sin and must repent to Allah. Mosques were established for prayer, so a person in a state of janabah should not enter them, out of reverence for Allah's sanctities and rituals.