The claim was then picked up by many (if not most) writers on the pyramids down to Richard Anthony Proctor, who used this single line of Proclus to argue for the Great Pyramid's astronomical function in several books of the 1870s and 1880s. He did not, however, provide a reference for Proclus. This has not stopped modern writers from John Anthony West to Alan Alford to Robert Bauval to Graham Hancock from quoting or citing Proctor's assertion, derived apparently from Greaves, that Proclus had declared the pyramid an observatory. All of these writers use this assertion in their works, and none quotes Proclus directly--only Proctor. I have never read the specific words of Proclus on this subject.
I have tried to track down the exact words of Proclus, and I have not been able to do so. I have scanned both a 19th century and 2007 translation of Proclus' commentary on Timaeus, and there does not appear to be any reference to the Egyptian pyramids in it. Nor did I find a reference to Sirius, or even much about astronomy. The closest I found in the Timaeus commentary, book 1, is when Proclus writes the following of the priests: "they survey without impediment the celestial bodies, through the purity of the air, and preserve ancient memorials, in consequence of not being destroyed either by water or fire."
I am frankly stumped. If anyone knows where this reference came from, outside of Greaves' imagination, I would be appreciative if you could leave a note in the comments.