Hebrews 12:15-16:

Looking diligently lest any man fail of the grace of God; lest any root of bitterness springing up trouble you, and thereby many be defiled;
Lest there be any fornicator, or profane person, as Esau, who for one morsel of meat sold his birthright.

See also:

Sickness as Chastening and Judgment in the New Testament
Beware of Pharisees