It was Lars' recommendation after the band unanimously decided to not cover any insanely popular Sabbath songs like: "War Pigs" or "Paranoid".

Album:
Garage Inc.

The cover of:
The song "Sabbra Cadabra" was originally released by Black Sabbath in December 1973 on the album "Sabbath Bloody Sabbath".

What Metallica says about it:
Lars: "I remember on the Load tour, talking with Kirk about this project one night in Frankfurt, staying up and watching the sun rise, and saying,'If we do any Sabbath, it's got to be 'Sabbra Cadabra'. There's actually a bit of that fill in '2x4'"

Lars also asserts: "Black Sabbath were not a direct influence on our music they had been going for too long. But they were a big influence on the four of us as we were growing up."

James: "You sit and listen to the Sabbath stuff and think 'wow that's great' and then you go to sing it, and you think 'wow I can't sing that high, we'll have to tune the song down a bit to get in that range'."