The First Slap

In the season 2 episode "Slap Bet", the gang tries to find out why Robin is so against going to the mall. Marshall and Barney have a bet riding on whether she got married in a mall or did porn and they ultimately agree to a slap bet where whoever wins gets to slap the other as hard as they can. Ultimately both guys were wrong in their guesses, and although both slapped one another pre-maturely, Marshall gives Barney the option to receive 10 slaps immediately or receive 5 slaps over a period of time. Barney chooses the five slaps and thus it all begins..