What should one do if he has prayed the Friday prayer in one mosque, then went to another mosque and found the Imam delivering the Friday sermon? Should he listen to the sermon and then pray two rak'ahs as a supererogatory prayer? And what should one do if he has prayed Fajr in congregation, and then a person who is late asks him to pray with him? And should he pray Maghrib as three supererogatory rak'ahs with another person?

If you pray Jumu'ah (Friday prayer) and then come to another mosque to pray Asr, and you find the Imam delivering the Jumu'ah sermon, it is recommended by the Shafi'is to repeat it. If you wish to sit and listen to both sermons, do not sit until you have prayed two rak'ahs as "Tahiyyat al-Masjid" (greeting the mosque prayer). After performing Fajr prayer, it is not prescribed for you to repeat it with someone else, due to the prohibition of prayer after Fajr until the sun rises. As for Maghrib prayer, there is no harm in repeating it with someone praying alone to attain the reward of congregational prayer, and you may repeat it as three or four rak'ahs.