He who wonders discovers that this is in itself a wonder.
Source, in full
Quote of Escher 1959 - in the introduction of M.C. Escher: The Graphic Work (1978) p. 8 - translated from the original book in Dutch: M.C. Escher - Grafiek en Tekeningen, publisher, Koninklijke Erven J.J. Tijl N.V. Zwolle, 1959