The common source of all modern quotes is probably Kline, Mathematical Thought from Ancient to Modern Times (1972), or Terence Tao, Solving Mathematical Problems, a personal perspective (1992), anyway, it is a genuine quote from Proclus' Commentary on the First Book of Euclid's "Elements", last lines from the first part of the Prologue.
Unfortunately, I don't have an English translation, the original Greek can be found here, page 46 starting from line 15, in English it would sound like this:
This is therefore the "mathesis" [we can translate it by general learning or education]: reminiscence$^*$ of the eternal ideas that are in the soul; and it is the reason why the study that helps us so well for the reminiscence of those ideas has taken the name of mathematics. And the function of this science is revealed by its name [which means to learn, as Proclus says, it is the study by which it is reached the general education]; that is, it sets in motion our innate knowledge, awakens the Intellect, purifies the thought, highlights the concepts that are in us in essence, eliminates the oblivion and ignorance we have from birth, loosens the bonds deriving from irrationality; following in this the god [maybe Hermes Trismegistus] who is the actual superintendent of this science, who brings to light our intellectual gifts, fills all things with divine reasons, moves souls towards the Intellect and awakens them as from a deep sleep, by means of research makes them reflect on themselves, perfects them through maieutic art, and with the discovery of the pure Intellect it also leads to a happy life.
Anyway, I suggest you to read the entire Prologue (and the whole Commentary, of course), which discusses in details the ideas on mathematics of Plato, Aristotle and the so called school of Pythagoras, an English translation is available.
$^*$ About mathematics as reminiscence, at page 45 Proclus cites Plato, Phaedo, section 73b ("When people are questioned, if you put the questions well, they answer correctly of themselves about everything; and yet if they had not within them some knowledge and right reason, they could not do this. And that this is so is shown most clearly if you take them to mathematical diagrams or anything of that sort") and 72e and Meno, section 82a.
