This article is from the Final-Fantasy and other Square Soft Games FAQ, by firstname.lastname@example.org (Nick Zitzmann) with numerous contributions by others.
Cid will clue you in as to what the code is should you mess up
initially. However, if you aren't able to get the code correct before
the time limit ends, you'll lose the Huge Materia for good.
Not going to bother with Cid's clues? In that case, the code (to be
entered one button at a time per message box) is: O, S, X, X.