Just a quick search, but it being an insta cast is at least mentioned by a poster on allakhazam:
http://everquest.allakhazam.com/db/item.html?item=564
Also mentioned on eqclerics.org (google cache):
http://webcache.googleusercontent.co...&ct=clnk&gl=us
There are also a couple of posts on allahs mentioning it with a 5 second cast time?
There is a post on eqcleric.org mentioning it has a 12 second cast?
P.S. While classic, I think allowing an instant clicky heal like that even retroactively is a bit crazy.