This phrase comes from a very funny
cartoon by
Sidney Harris.
A senior professor is reviewing the chalkboard of his junior colleague. He says, "I think you should be more explicit here in step two." He is pointing to a section of the board that says, in the middle of some complicated-looking equations, 'THEN A MIRACLE OCCURS...'.
It's available on a t-shirt at http://www.sciencecartoonsplus.com/gallery.htm ;
the cartoon itself is available in a close-up at http://www.aip.org/education/sps/miraclen.jpg .