I read CoB and I forgot where to find the part where Jace says The Law is hard but it is the Law in Latin.

"Sed lex dura lex," 発言しました Jace automatically. "'The Law is hard but it is the Law'." - Page 104

That's the page off the Australian paperback copy. So it could be different depending on which country you're from. It's at the back of Chapter 7 - The Five-Dimensional Door if the pages don't match up. This way あなた can find it yourself in your own copy.

It's page 108 in the American hard-back version. It's the same quote is the same chapter as mentioned above.