Imam Jafar al Sadiq (as) was asked that can one take names of 12 Imams (as) in prayer.
Imam (as) replied : Yes, precisely.
Imam Jafar al Sadiq (as) said : If one says that There is no God but Allah, Muhammad (saww) is the Messenger of Allah, then he has to say Ali (as) is the prince of faithful and vicegerent of Allah.
Divine Treasure 