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.