Is it recommended to go out to the mosque for prayer when it rains, or is it permissible?

There is no harm in going out in the rain as long as it does not cause harm to the person. The Prophet, peace and blessings be upon him, used to expose himself to it and say: "It has just come from its Lord." If the rain is heavy, it is considered a concession for not attending Friday prayer and congregational prayers. Taking concessions is better, as the Prophet, peace and blessings be upon him, said: "Indeed, Allah Almighty loves that His concessions be taken, just as He loves that His firm commands be taken." Leaving off taking a concession is disliked, due to the element of pushing away arrogance and elevating oneself above accepting what the Sharia has permitted.