It wasn't a dream. It was Paxcow. Paxcow says Daddy's gonna do something really bad.
10 seconds sound clip from the Pet Sematary (1989) movie soundboard.
You can hear this line at 01:03:39 in the Blu-ray version of the movie.
Quote context
[...]
- They don't have to know. Rachel and Ellie don't ever have to know.
- Mommy. Mommy. Mommy! Mommy. Mommy! Oh, Mommy!
- Honey, you just had a bad dream, that's all. You know that, don't you?
- It wasn't a dream. It was Paxcow. Paxcow says Daddy's gonna do something really bad.
- Who is this Paxcow? Is he like the bogeyman?
- He's a ghost. He's a good ghost. He says he was sent to warn us.
- He says he was near Daddy because they were together when his soul was dis... Dis I can't remember.
- There are no ghosts, Ellie. There are no ghosts.
- Now I want you to go to sleep and forget all this nonsense. Do you hear me?
[...]