There is not a direct or written source for this phrase, but as Einstein used to say (Picasso really loved this phrase): "The secret to creativity is knowing how to hide your sources"
As a matter of fact Picasso said once: “Good artists copy, great artists steal. ". This is the same phrase by T.S.: Elliot: " Immature poets imitate; mature poets steal". Who said it first?
Picasso hardly meant that great artists steal popular designs whose original source is known to everyone. What Picasso did mean was that great artists rummage through the great junk heap of lost, bypassed, and forgotten ideas to find the rare jewels, and then incorporate such languishing gems into their own personal artistic legacy.
Picasso was a genius, no doubt about it, but at the same time it was a "con" man, artistically speaking: his artworks are full of copies from other artist works, in his own style, pf course, and that makes him a genius.
Back in business, in Spanish there is nothing similar to this phrase, except for the quotation "Si lo puedes imaginar, lo puedes lograr" (“If you can imagine it, you can achieve it") attributed to Einstein, but, curiously, he never said or wrote that!
Probably, the phrase is not entirely his, but who knows where or from whom he took it!
For sure, the whole idea is taken from Socrates: physical objects and physical events are "shadows" of their ideal or perfect forms, and exist only to the extent that they instantiate the perfect versions of themselves. That can explains the birth of Cubism.
These three quotations have the same meaning:
-“If you can imagine it, you can achieve it; if you can dream it, you can become it.” William Arthur Ward
-"Your imagination is your preview to life's coming attractions." Albert Einstein.
-"Everything you can imagine is real"- Pablo Picasso.
Who said it first, impossible to say. All of these three men were contemporaries.