Numbers 14:36-37:

And the men, which Moses sent to search the land, who returned, and made all the congregation to murmur against him, by bringing up a slander upon the land,
Even those men that did bring up the evil report upon the land, died by the plague before the LORD.

Notes on Numbers 14:36-37:

We pay too little attention to the power of the tongue.  The men who were smitten by a plague before the Lord died because of something they said!  They did not commit adultery or murder anyone.  Their sin was twofold: They chose not to believe God’s explicit promise of taking Canaan, and then they spoke unbelief to incite others to unbelieve along with them!  They paid for their unbelief with their lives.  God takes unbelief more seriously than we do!  Unbelief (like gossip or complaining) is socially acceptable today, but it has never been acceptable to God, before whom everything that does not arise from faith is sin (Romans 14:23).

Thank God, we live in the day of grace, but let’s not tempt God through unbelief.  Paul said that these Old Testament accounts were written to warn us so that we do not fall into the same sins.  Just as unbelief kept Israel out of Canaan, it can keep you out of God’s best in your life.

See also:

Sickness as Chastening and Judgment in the Old Testament
Objection: If Proverbs 18:21 Meant What Faith Preachers Say It Does, Old Testament People Could Have Gone Around Speaking Health to the Sick and Death to Cancer and Other Diseases Before Jesus Came