Verses 67 to 71 of John chapter 6. 

Then Jesus turned to the Twelve and asked, “Are you also going to leave?”

Simon Peter replied, “Lord, to whom would we go? You have the words that give eternal life. We believe, and we know you are the Holy One of God.”

Then Jesus said, “I chose the twelve of you, but one is a devil.” He was speaking of Judas, son of Simon Iscariot, one of the Twelve, who would later betray him.