Is it permissible to invoke blessings upon the Prophet (peace be upon him) with the phrasing: "O Allah, send Your blessings and peace upon you, O Messenger of Allah," or "Prayers and peace be upon you, O Messenger of Allah," if the speaker is not in the Prophet's Mosque?

It is best for a Muslim to adhere to the reported formulas for sending prayers upon the Prophet. It is permissible to use a formula addressed to him in the vocative case, such as "Peace be upon you, O Prophet." This is not a direct address to the Prophet, but rather due to the strength of the person invoking him, as if he were before them. It is not a condition for the person to be in the Prophet's Mosque.