It's in season 3, episode 6 "Red Sky at Morning". During a summoning ritual Sam mentions Castiel and other angels. The funny thing about this is that it is an early episode of season 3 while Castiel's first appearance in the series is in the first episode of season 4.
I have noticed this early but thought this time I could upload it :).
Sam seems to say, "Aziel, Castiel, Lamisniel, Rabam, Erlei, and Belam".