When I was a kid I heard the following story at least a few times in grade school from the school headmaster (probably around Rosh Hashanah time). It's based on a Jewish legend or midrash, and I'd like to know the source. (It's very apropos for some things some friends of mine and I are dealing with), but Google and Google books searches are failing me, so I'm hoping someone can tell me where this story is from.
Details may be a bit off since it's been a couple of decades since I heard it.
Two angels were arguing in heaven about who was stronger - a good angel and a bad angel (or perhaps the yetzer hatov and the yetzer harah). The bad angel boasts that in a single instant he could undo years of the good angel's work. They decide to test this out. They find a human being, and the good angel has 3 years to do whatever he wants. The man lives in a kingdom where the king wants the milk of a lion, and will pay a considerable reward for it. (The usual Jewish legend stuff - probably the hand of his daughter and half his kingdom). The man, with the assistance of the angel, goes through all sorts of travails and manages to secure the milk of a lion. He returns after years of adventure with the milk in a jug, and presents it to the king. Just as he opens his mouth the 3 years end, and the bad angel takes over, causing the man to say "I have brought you the milk of a... sheep!" The king is angry and orders the man executed. Somehow the man manages to talk his way out of it, but the lesson (or at least the lesson that we were supposed to learn) is that even a moment of indiscretion can destroy a lifetime of good works.
If anyone recognizes this story (some details may be off) and can tell me where it's from, I'd be much obliged.