The two links in my comment, Google Books and Project Gutenberg, are different editions of the same translation by Thomas Taylor (1818). The Gutenberg edition has a different layout, with endnotes instead of footnotes. The scan of the actual book shows they were originally footnotes. It is in the footnote by Thomas, where we find Syrianus quoted:
Pythagoras, therefore, in the Sacred Discourse, clearly says, that “number is the ruler of forms and ideas, and is the cause of Gods and dæmons.”
I wasn't sure whether or whom Thomas was quoting at first, but the Greek is also given and more clearly attributed to Syrianus. One can find the Greek in the following manuscript:
Syrianus, In metaphysica commentaria (Cambridge, Trinity College, MS O.9.4),
https://cudl.lib.cam.ac.uk/view/MS-TRINITY-O-00009-00004/146,
p. 132, ll. 22-24.
αυτος μεν Πυθαγορας, εν τῳ ιερῳ λογῳ, διαρρηδην μορφων και ιδεων κραντορα τον αριθμον ελεγεν ειναι, και θεων και δαιμονων αιτιον.
Remarks on the manuscript by the Cambridge library:
This manuscript, probably copied in the last thirty years of the 17th century, is a copy of the commentary on Aristotle's Metaphysics by the 5th-century Neoplatonist philosopher Syrianus.
...
The manuscript was probably a copy of Oxford, Bodleian Library, MS Auct. T.1.16 (Misc. 194).
The text posted below the image is the beginning of the Greek quote in the footnote from the Gutenberg transcription. It seems to be missing the first three words from the manuscript:
πῶς δ'ἄν
My Greek is pretty poor. Perhaps someone else would help. Literally the words translate "And how [or thus, as] if...", where "and" (δε) often combines with μεν (second word in Taylor's quote) to form a contrasting pair of phrases. Taylor includes "therefore" in his translation, which may come from these words. Since ἔλεγεν seems to be the indicative mood, it's unlikely that the Greek means "as if Pythagoras said...."
In sum, the source of the purported quote seems to be Syrianus (4th/5th c. CE, about 100 years after Iamblichus and 900 years after Pythagoras), who attributes it to something Pythagoras himself expressly said (διαρρήδην ἔλεγεν). The English version is from Taylor's translation. As Lee Mosher points out, the text cited by Syrianus (τῷ ἱερῷ λόγῳ, the Sacred Discourse) has not been found, and doubt surrounds whether such a text ever existed. Interestingly, Iamblichus questions whether Pythagoras actually wrote the Sacred Discourse in the passage leading up to Thomas's footnote.
