Answer
In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
In this fatwa:
It is not allowed to leave the call unanswered or to leave the mosque after it has been made, unless there is some excuse or one has the intention to return for the prayer.
In his famous work, Fiqh As-Sunnah, Sheikh Sayyid Sabiq, elaborates on the subject as follows:
It is not allowed to leave the adhan (call to prayer) unanswered or to leave the mosque after it has been made, unless there is some excuse or one has the intention to return for the prayer.
Abu Hurairah (may Allah be pleased with him) related that the Prophet (peace be upon him) told them, “If one of you is in the mosque and the adhan is made, he should not leave the mosque until he prays.” (Related by Ahmad with a sahih chain)
It is also related that Abu Hurairah said about a man who left the mosque after the call had been made, “That man has disobeyed Abu al-Qasim (the Prophet, peace be upon him).” This is related by Muslim and others.
Mu`azh at-Jahni related that the Prophet said, “It is the utmost apathy and sign of disbelief and hypocrisy that one who hears the call of Allah to salvation does not respond.” (Related by Ahmad and at-Tabarani)
Commenting upon this, at-Tirmizhi says, “It has been related from more than one of the companions that one who hears the call and does not respond will have no prayer. Some said that this is the maximum imposition, which shows that there is no excuse for one who does not attend the congregational prayer without a valid reason.”
Almighty Allah knows best.