5x05 "The One with the Kips"

Chandler and Monica go away for the weekend but spend the entire time fighting after Monica insists on constantly changing rooms due to her high standards whilst Chandler tries to watch a car chase on TV. Ross tells Rachel about his agreement with Emily, and he and Rachel get into a fight. After Monica tells Rachel that she's lost her eyelash curler, Joey realizes that Monica and Chandler are in a relationship (as the manager of the hotel in which Chandler stayed in had informed Joey that there was an eyelash curler left in Chandler's room). He is happy for them but shocked, and begrudgingly agrees to keep the secret.