Here is a rectangle made out of 2x1 dominoes:

[![2x1 dominoes arranged into a rectangle][1]][1]

It can be divided along a line into two smaller rectangles:

[![Splitting the big rectangle into two smaller rectangles][2]][2]

What is the smallest (in area) rectangle of (edit) multiple 2x1 dominoes that *cannot* be split along a line into two smaller rectangles?

  [1]: https://i.sstatic.net/NSoEV.png
  [2]: https://i.sstatic.net/TG8HP.png