Is it permissible to go to a mosque said to have been built by Amr ibn al-Aas just to see it?

There is no legal impediment to visiting any mosque built by the Companions or others, or any other historical site, with the intention of reflection and observation, unless there is another legal impediment, such as intending it for worship. For mosques are equal in virtue, except for the three mosques: al-Masjid al-Haram, al-Masjid an-Nabawi, and al-Masjid al-Aqsa, as the Prophet, peace and blessings be upon him, said: "Journeys are not undertaken except to three mosques: al-Masjid al-Haram, the Mosque of the Messenger of Allah, peace and blessings be upon him, and al-Masjid al-Aqsa." Therefore, it is not permissible to travel for the purpose of worship in mosques except to the three aforementioned mosques. And whoever vows to fulfill his vow, or pray, or perform i'tikaf, can do so in the nearest available mosque, without needing to travel to mosques other than the three.