This seems to be a popular rumor on the Internet, especially in China. We didn't find any traces of the motto on the walls of Harvard Library. Professor robert darnton said unequivocally, "As a university librarian, I can prove that there are no such words on any walls of 73 libraries in Harvard." (WSJ, "A generation of China students got their inspiration from a scam about Harvard." 1 1 month 15, 20 12)
What I found on the Internet is this:
If not, please enjoy the pain. Please enjoy the inevitable pain.