The source is R' Moshe Feinstein zatzal based on the assumption that pizza is pas habaah b'kisnin. The halacha of pas habaah b'kisnin it that one makes mezonos on it if eaten as a snack and hamotzi if eaten as a meal and presumably a single slice of pizza is not a full meal, however if it eaten with side dishes it could be.
The classification of pizza as pas habaah b'kinsnin, however is problematic. It is not based on the cheese on top, because the Magen Avraham and most poskim hold that cheese and other non-sweet filings do not render the baked item pas habaah b'kisnin and it would be hamotzi like any other bread. Even if the pizza is made with milk or juice in place of water, it is still questionable if it is sweet enough to lose its status as bread. But most importantly, pas habaah b'kisnin by definition is an object that most people most of the time would not eat as a meal. Since I don't think this is true regarding pizza, it would be considered bread l'halacha and one would make hamotzi on any quantity even if made without water.
Therefore, in my opinion all pizza is always hamotzi unless one lives in a place where most people usually eat pizza only as a snack.