Don Quixote Volume 1, Chapter 24
They exchange words of elegant courtesy and Don Quijote tells him that he is a knight errant and if he will tell him what brought him to such a state, he will do what he can to help or commiserate with him. After studying Don Quijote, he asks for food (almost swallows it whole) and then beckons for them to join him as he sits on the grass. He promises the complete tale, but only on the condition that they do not interrupt or ask questions.
His name is Cardenio and he is of a wealthy noble family. He was just about to become officially engaged to a beautiful maiden named Luscinda (from a family similar to his) whom he had known and loved since childhood and who returned his feelings in kind. Both families favor the union, and Cardenio had already asked Luscinda's father for her hand but told Cardenio that the request must come from his father. Just as he was about to approach his father to make this request for him, a Grand Duke summoned him to act as companion for his eldest son, and so he asked Luscinda's father to delay giving her hand in marriage to anyone else till he could shortly return and make the appropriate arrangements.
After arriving at the Duke's home, friendship blossoms between himself and the younger son, Don Fernando, who confides in Cardenio that he has promised a beautiful farmer's daughter that he would marry her (though he has no intention of fulfilling this promise now that he has slept with her). He is now eager to leave town for a while and pretends he would like to shop for horses with Cardenio in his neighborhood -- where he meets the lovely Luscinda who ignites his ardor, although she is all but engaged to his friend.
During the course of Cardenio telling his story he mentions something about the book, Amadis of Gaul, and Don Quijote is off and running his mouth about knight errantry and whatnot. Cardenio insults a character (that is a queen in a book), by saying he believes she was having sex with her physician; this insults Don Quijote who insults Cardenio by calling him a liar and a rascal. Cardenio smashes a rock into Don Quijote's chest, and jumps up and down on Sancho's chest and beats the goatherd when they try to help Don Quijote. After the madman leaves, Sancho tries to blame the goatherd for what happened, saying he should have warned them of the man's violent temper (even though the goatherd had warned them).