Is it permissible to make the intention of Iʿtikāf in the mosque before entering it with the phrasing: "I intend to perform Iʿtikāf in the جامع (mosque) for the entire duration of my stay therein"?

The questioner's brother may intend i'tikaf (spiritual retreat) during this period, and his i'tikaf is valid according to the Shafi'i school, because they hold that i'tikaf is valid even for a short time. In fact, some of them believe that whoever enters the mosque for prayer or other purposes should intend i'tikaf for the duration of his stay in it. It is a condition that the intention accompanies the entry into the mosque.