How to show a Message in posts that belongs to a single category or two?

Hi,
I want to show a message right after the post if that post belongs to a certain category (or categories).

I know I need to put that within the loop, but what’s the code?

I tried,

<?php
if ( in_category( '11' )) {
 // My Message
}
?>

and

<?php if ( in_category( array( '11', '12', '13') )) {
	// My Message
}
?>

What am I doing wrong?