Is it permissible to perform tayammum inside the mosque for Friday prayer—especially in the first rows—for fear of losing one's place, when water is available?

The Shari'ah has permitted Tayammum (definition) (dry ablution) when water is unavailable or one is unable to use it, based on the Almighty's saying: "And you find no water, then perform Tayammum with clean earth." If water is present and there is no other legitimate reason for Tayammum, then Tayammum is not permissible. Water purification is a condition for the validity of prayer for those who are able to perform it. Preserving prayer in the first row does not justify abandoning purification. A person can leave the mosque to perform ablution and then return to their place, for whoever precedes to a place in the row has a greater right to it.