From the book of Jude:
1:5 I will therefore put you in remembrance, though ye once knew
this, how that the Lord, having saved the people out of the land of Egypt,
afterward destroyed them that believed not.
1:6 And the angels which kept not their first estate, but left their own
habitation, he hath reserved in everlasting chains under darkness unto the
judgment of the great day.
1:7 Even as Sodom and Gomorrha, and the cities
about them in like manner, giving themselves over to fornication, and going
after strange flesh, are set forth for an example,
suffering the vengeance of eternal fire.
Other quotes:
"The flames that consumed the cities of the plain shed their warning light down even to our time. We are taught the fearful and solemn lesson that while God's mercy bears long with the transgressor, there is a limit beyond which men may not go on in sin. When that limit is reached, then the offers of mercy are withdrawn, and the ministration of judgment begins." (Patriarchs and Prophets, pg 162)
"In every age there is given to men their day of light and privilege, a probationary time in which they may become reconciled to God. But there is a limit to this grace. Mercy may plead for years and be slighted and rejected; but there comes a time when mercy makes her last plea. The heart becomes so hardened that it ceases to respond to the Spirit of God. Then the sweet, winning voice entreats the sinner no longer, and reproofs and warnings cease." (Desire of Ages, pg. 587)