What was secret message/sign that Houdini sent to his wife AFTER his death?

Question

Houdini wanted to either prove or disprove life after death. To accomplish this he and his wife came up with a message of some kind that ONLY they knew. Upon the first ones death they were supposed to get the secret message back to the still living spouse. This would prove life after death AND that some psychics were real. I have read that his wife eventually DID get the secret message but little else about this. What was the message and what else do people know about this part of Houdini history???

Answer 1

Here it is..I had heard it before, but couldn't remember it.

http://people.howstuffworks.com/ghost-st...

You have to scroll down.