Quote from Marshall in Slapsgiving
Future Ted: [v.o.] I should explain. Kids, you remember the slap bet. A year earlier, Marshall made a bet with Barney and won the right to slap him five times. He had already used the first and the second. But Marshall had something special in store for the third. [flashback:] Marshall: [on the phone with Barney] slapcountdown.com. Barney: No!