Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) narrates: The Noble Prophet (peace and blessings be upon him) said: "Whoever hears the call to prayer and does not respond to it (i.e., does not participate in the congregational prayer), his prayer is not valid, except if he has an excuse, for the ruling regarding an excuse is different." (Imam Ibn Hibban, may Allah have mercy on him, says:) In this narration, there is evidence that the command of the Noble Prophet (peace and blessings be upon him) to participate in the congregation is an obligatory command, not merely a recommendation. Because if the intent of the Prophet’s statement, "his prayer is not valid except for the one who has an excuse," was to indicate virtue, then when an excused person prays alone, he would attain the virtue of congregation. Since this is impossible, it is established that the command to come for congregation is obligatory, not merely recommended. As for the excuse due to which a person is considered excused from participating in the congregation, I have researched all the hadiths on this matter and found that the hadiths indicate that this excuse can be ten different things.