《太平广记·卷四百七十·水族七》
等到分手时，谢二把一封信交给他说： 我家住在魏王池的东面。到了池边，你就敲大柳树，我家里的人如果出来，你把信交给他，就可以得到钱了。 
及别，以书付之曰： 我宅在魏王池东，至池，叩大柳树。家人若出，宜付其书，便取钱也。 
When parting, Xie Er handed him a letter and said: My house is to the east of Wei Wang Pond. When you arrive at the pond, knock on the big willow tree. If someone from my family comes out, give him the letter and you can get the money. 