Is it permissible for people to retain spaces in the mosques when they go outside to attend to their needs? There is a hadîth where the Prophet says: “A man has more right to his sitting place. If he leaves from it to fulfill his needs and then returns, then he has more right to it.” [Sunan al-Tirmidhî (2751)] Does this apply in the mosque?
When someone is inside the mosque, it is not wrong for him to reserve his space. It makes no difference whether this space is in the first row or elsewhere.
Likewise, if he is sitting in a space and has to go out of the mosque for some legitimate reason, such as to perform wudû’, then he does indeed have the right to reserve his space for his return.
However, the person will not be allowed to retain a place in the mosque and then go out for some worldly reason or to have a sleep. He may also not do so and then leave for a long stretch of time. In such a situation, others will have the greater right to that place.