It is in Book 8. I edited the original post to make this clear.
The George Long translation reads:
For nothing should be done without a purpose.
My Latin version (I don't know who translated it) reads:
Nihil enim temere faciendum.
I'm not sure which version is the most accurate, since I can't understand Koine Greek.
Once again, here's the new thread for posting quotes, with the usual rules:
Please post all quotes separately, so that they can be voted up/down separately. (If they are strongly related, reply to your own comments. If strongly ordered, then go ahead and post them together.)