The ruling on one who prayed the midday prayer in a mosque and then caught a unit of prayer in another mosque

Question : A man prayed the midday prayer in congregation, then he went to another mosque where they were praying the midday prayer, and he caught a unit of prayer with the imam. Is it permissible for him to pray a unit, and do you tell him it is a greeting to the mosque, or is he required to follow the imam? The apparent meaning of the hadith is that he must complete four units, based on the saying of the Prophet (peace and blessings be upon him) unless there is a reason, such as if he is hurrying to attend a funeral and fears that completing the four units will cause him to miss his purpose.

Answer : If he fears that his purpose will be missed, there is no harm in his praying a unit, because he caught a unit; he prays a unit and says the closing salam.