Verses 60 to 66 of John chapter 6. 

Many of his disciples said, “This is very hard to understand. How can anyone accept it?” Jesus was aware that his disciples were complaining, so he said to them, “Does this offend you? Then what will you think if you see the Son of Man ascend to heaven again? The Spirit alone gives eternal life. Human effort accomplishes nothing. And the very words I have spoken to you are spirit and life. But some of you do not believe me.” (For Jesus knew from the beginning which ones didn’t believe, and he knew who would betray him.) Then he said, “That is why I said that people can’t come to me unless the Father gives them to me.” At this point many of his disciples turned away and deserted him.