What does one do if he prayed at home thinking the congregation had finished?

Question:
A man woke up from his sleep, got up, and prayed the Fajr prayer, thinking that the mosque had already prayed. When he finished his prayer, he heard the mosque calling for the iqamah?

Answer:
He should go and pray with them.
Q: And what about the one who had already prayed?
A: His prayer with them becomes a voluntary prayer, and the first prayer was the obligatory one.
Q: But what if he did not pray with them?
A: The more apparent and closer view is that it suffices for him, but if he hastens to pray with them, that has merit—if Allah wills.
Q: But what reward of congregation is recorded for him?
A: The reward of congregation in a voluntary prayer.