Where does it say that the Son of Man will come at an hour you do not expect?

The exact words occur in Matthew and Luke:

Therefore you also must be ready, for the Son of Man is coming at an hour you do not expect. (Matthew 24:44; also Luke 12:40)

We are told that Jesus Christ will come unexpectedly, although not secretly, in Matthew 24:42-51; Matthew 25:13; 1 Thessalonians 5:2-6; Revelation 3:3 and Revelation 16:15.