1785/1 - Abu al-Sha‘thā’ reported: “As we were sitting in the mosque with Abu Hurayrah (may Allah be pleased with him), the Muezzin proclaimed the Adhān and then a man stood up in the mosque and started walking out. Abu Hurayrah followed him with his eyes until he went out of the mosque. Thereupon, he said: ‘As for this man, he has disobeyed Abu al-Qāsim (may Allah’s peace and blessings be upon him).’” [Narrated by Muslim]
1) It is disliked to leave the mosque after the Adhān for those who are obligated to offer the prayer, except for a valid excuse, lest people may think ill of him.
2) It shows the merit of attending the congregational prayer and how the Shariah is keen that it should not be missed.
Benefit:
In an authentic Hadīth, the Prophet (may Allah’s peace and blessings be upon him) said: “No one hears the Adhān in this mosque of mine and then leaves it for some need and does not return except for a hypocrite.” [Narrated by Al-Tabarāni in Al-Awsat]
Although the Hadīth speaks specifically about the Prophet’s Mosque, the meaning applies to all mosques, based on the numerous Hadīths proving the obligation of attending the congregational prayers, and if a person leaves the mosque, without a valid excuse, he may miss this obligation, and Allah knows best.