Actually, the source was Castenada.
From "The Art of Dreaming" 1993:
“Sit here and visualize their shape until they are just like they are in your dreams,†don Juan said in my ear. “Let me know when you have them in focusâ€.
It took me very little time and effort to have a complete mental picture of their shape, just like in my dreams. It did not surprise me at all that I could do it. What shocked me was that, although I tried desperately to let don Juan know I had pictured them in my mind, I could not voice my words or open my eyes. I was definitely awake. I could hear everything.
I heard don Juan say, “You can open your eyes now.†I opened them with no difficulty. I was sitting cross-legged on some rocks, which were not the same ones I had felt under me when I sat down. Don Juan was just behind me to my right. I tried to turn around to face him, but he forced my head to remain straight. And then I saw two dark figures, like two thin tree trunks, right in front of me.
I stared at them openmouthed; they were not as tall as in my dreams. They had shrunk to half their size. Instead of being shapes of opaque luminosity, they were now two condensed, dark, almost black, menacing sticks.
Not the exact passage I was looking for, but close enough. Many people consider Castanada's books to be pure fiction but I actually had some success using his techniques for becoming more aware in the dream state. The problem is, there is no way to prove their efectiveness, all the evidence is anecdotal.