Matthew 24:50
that servant's master will come on a day when he doesn't expect him and at an hour that he doesn't know.
Matthew 24:42-44
"So keep on watching, because you don't know on what day your Lord is coming.
1 Thessalonians 5:2-3
for you yourselves know very well that the Day of the Lord will come like a thief in the night.
Revelation 3:3
So remember what you received and heard. Obey it, and repent. If you are not alert, I will come like a thief, and you won't know the time when I will come to you.
Proverbs 29:1
After many rebukes, the stiff-necked man will be broken incurably, without any warning.