What is the ruling on visiting the Prophet's grave without intending to?

Visiting the Prophet's grave (peace and blessings be upon him) is permissible for those who are in Medina or close to it. As for those who are far away, it is not permissible for them to travel specifically to visit his grave, due to the Prophet's (peace and blessings be upon him) saying: "Journeys are not undertaken except to three mosques: Al-Masjid Al-Haram, the Messenger's Mosque (peace and blessings be upon him), and Al-Masjid Al-Aqsa."