How to reference a quote when I don't know the page number?

I want to use a quote from my textbook in an essay. If a a textbook cites a quote from another author, I tend to cite that author rather than the textbook, but I can't find the source of the quote and therefore I don't know what the page number is. Should I just leave it out? My course uses the Harvard referencing format

There's a protocol for secondary citations... e.g. p 14 of the anglia ruskin ref guide
https://libweb.anglia.ac.uk/referencing/files/Harvard_referencing_2015.pdf

the reference list should only contain the works that you've actually read