What is the ruling on invoking blessings and peace upon the Household [of the Prophet] or parents or any other person, given that such blessings were ordained for the Prophet, peace and blessings be upon him?

It is sunnah to pray upon the household of the Prophet, peace and blessings be upon him, as in the Tashahhud: "O Allah, send prayers upon Muhammad and upon his household and his wives." One does not pray upon anyone other than the Prophets except in conjunction with them, because prayer is specific to the Prophets, just as "Azza wa Jall" (Mighty and Majestic) is specific to Allah. So, it is not said: Muhammad, Mighty and Majestic, nor Abu Bakr, peace and blessings be upon him.