It's not individual dreams. The Fade is a realm where your consciousness goes when you dream. Entering it physically is... nearly impossible [ with perhaps the exception of Corypheus and the Inquisitor, for all he knows. ]. Magic is... drawn from that place. Not from people's dreams, though some mages can manipulate their own dreams when they sleep.

[ Though he is not a mage, as a templar, he was educated on some of the basic theory behind it, of course. ]