Apparently this is the passage in question (Enneads, IV, iv, 40):
But magic spells; how can their efficacy be explained?

By the reigning sympathy and by the fact in Nature that there is an agreement of like forces and an opposition of unlike, and by the diversity of those multitudinous powers which converge in the one living universe.

There is much drawing and spell-binding dependent on no interfering machination; the true magic is internal to the All, its attractions and, not less, its repulsions. Here is the primal mage and sorcerer- discovered by men who thenceforth turn those same ensorcellations and magic arts upon one another.

Love is given in Nature; the qualities inducing love induce mutual approach: hence there has arisen an art of magic love-drawing whose practitioners, by the force of contact implant in others a new temperament, one favouring union as being informed with love; they knit soul to soul as they might train two separate trees towards each other. The magician too draws on these patterns of power, and by ranging himself also into the pattern is able tranquilly to possess himself of these forces with whose nature and purpose he has become identified. Supposing the mage to stand outside the All, his evocations and invocations would no longer avail to draw up or to call down; but as things are he operates from no outside standground, he pulls knowing the pull of everything towards any other thing in the living system.

The tune of an incantation, a significant cry, the mien of the operator, these too have a natural leading power over the soul upon which they are directed, drawing it with the force of mournful patterns or tragic sounds- for it is the reasonless soul, not the will or wisdom, that is beguiled by music, a form of sorcery which raises no question, whose enchantment, indeed, is welcomed, exacted, from the performers. Similarly with regard to prayers; there is no question of a will that grants; the powers that answer to incantations do not act by will; a human being fascinated by a snake has neither perception nor sensation of what is happening; he knows only after he has been caught, and his highest mind is never caught. In other words, some influence falls from the being addressed upon the petitioner- or upon someone else- but that being itself, sun or star, perceives nothing of it all.