"When there is no more room in hell...

the dead will walk the Earth."

What is this quote from? A movie or sci-fi book?

"Dawn of the Dead"--1978
George Romero's follow up to "Night of the Living Dead".

Actually, I think it was spoken in "Day of the Dead". May be in both.

Not in Day--just in Dawn.
Day had some good ones with my fave spoken by Mary Jo:
"This ain't a god damned field trip people, this is a Fu....g War!!