I recall this story about a rebbe who is given a Sefer by some Malach. Or maybe he wrote it himself, I don't remember. He tells his chassidim that if any of them copy it, they will be sent to gehinom. The next day he asks his chassidim if any of them had copied the book. They were obedient and none of them had. "Such an important book!" The rebbe remarked. "Is there no mesiras nefesh in this generation?"
Who is this story about?