Ma'arif al-Qur'an Tafsir: Al-Yinn Ayah 18

وَأَنَّ الْمَسَاجِدَ لِلَّهِ فَلَا تَدْعُوا مَعَ اللَّهِ أَحَدًا (...and that masajid (mosques) belong to Allah; so, do not invoke anyone along with Allah....72:18). The word masajid is the plural of masjid. Here the word could be taken in its popular sense, that is, mosques or places of worship dedicated for the performance of prayers. In this case, it would mean that all mosques belong to Allah, dedicated to His sole worship and therefore we are not permitted to call on anyone else besides Allah, like the Jews and Christians commit shirk in their places of worship. In sum, the mosques must be kept clear of all false beliefs and vile deeds.

The word masajid could also have another sense. It could be the plural of masjad, with the letter jim carrying fath, in which case it would be masdar mimi 'infinitivity' and mean 'to prostrate or prostration'. The verse in this sense would signify that worship is reserved exclusively for Allah. It is not permitted to prostrate to anyone, because if he calls on anyone else for help, it is as though he is prostrating to him which must be avoided.