However, it is obligatory upon whoever can reach the Masjid to go to offer Salah in congregation. This is according to the legal evidence. The Prophet (peace be upon him) says, Whoever hears the Adhan (call to Prayer) and does not respond to it, their Salah is not accepted, unless he has an excuse. Ibn `Abbas (may Allah be pleased with them both) was asked, "What is a valid excuse?" He replied, "Fear or illness." (Related by Ibn Majah, AlDaraqutny, while Ibn Hibban and AlHakim ranked it (the Hadith) as Sahih (authentic),with a sound Isnad (chain of narrators))