Sam wants to prevent Anna from carrying out her mission on the episode "The Song Remains The Same." That's because her mission is to stop Sam from ever being born.
Julie McNiven is seen here as Anna. She plays a key role on the episode "The Song Remains The Same," aiming to kill Sam's parents before they conceive him.
Dean: Where's Cass? Chuck: He's dead. Or gone. The archangel smote the crap out of him. I'm sorry. Dean: You're sure? I mean, maybe he just vanished into the light or something. Chuck: Oh, no. He exploded. Like a water balloon of chunky soup.