There is a proof without words in [Étienne Ghys' ICM address][1] and [a short proof with words by Rubinstein and Gardiner][2] (Compositio, 1979).


  [1]: http://ghys.perso.math.cnrs.fr/exposes/icm.pdf
  [2]: http://archive.numdam.org/item/CM_1979__39_3_297_0/ " A note on a 3-dimensional homogeneous space"