Matthew 24:50
"The lord of that servant shall come in a day when he looketh not for him, and in an hour that he is not aware of,"
Cross References (ranked by relevance)
Revelation 3:3
Remember therefore how thou hast received and heard, and hold fast, and repent. If therefore thou shalt not watch, I will come on thee as a thief, and thou shalt not know what hour I will come upon thee.
Proverbs 29:1
He, that being often reproved hardeneth his neck, shall suddenly be destroyed, and that without remedy.