How I Met Your Mother S02E09 - Slap Bet

In the year 2030, Ted Mosby (voiced by Bob Saget) gathers his daughter and son to tell them the story of how he met their mother.
When Robin refuses to go to the mall, Marshall and Barney propose a "slap bet". Marshall believes she was married, and Barney believes she was a porn actress. Robin reveals she was in an embarrassing music video in the mall. Marshall is eventually awarded five slaps to be delivered anytime, and lands his first slap.