The Spaniard confessed he told Fernando Teixeira that he was free to go out and party to celebrate Pep Guardiola's side's progression in the Spanish cup
Fernando Teixeira controversially chalked off a Sergio Ramos goal, and went on to dismiss the former Sevilla defender with a second yellow card in the dying moments of the game.
As the game concluded, Casillas approached Teixeira and proclaimed: "Texeira, now you can go out and party with Barca!"
Speaking in the wake of the match, the 30-year-old attributed the insult to the passion the clashes between the two sides stir up.
"It was in the heat of the moment. You feel powerless when he looks the other way. I even got a yellow card when I tried to separate the players [during a first half scrap]," Casillas said according to Marca.
Despite the defeat, the Spain captain said that he was proud of his team-mates after they came back from a two-goal deficit.
"Madrid can be proud of its team. Now we have two competitions ahead."
Starting in November and continuing through to the middle of February, Goal.com is publishing exclusive extracts from Graham Hunter's highly-anticipated new book, 'Barca: The Making of the Greatest Team in the World'. Friday's extract looks at how Barca suffered rare disappointment at the hands of Real Madrid in a gripping finale to last season's Copa del Rey. Don't miss it!