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Only cite sources you've actually seen. In this case, it sounds like you have a secondary source (your link) that quotes from and does not cite a primary source. All you can say with certainty is...
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#3: Attribution notice added
Source: https://writers.stackexchange.com/a/40481 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
#2: Initial revision
Only cite sources you've actually seen. In this case, it sounds like you have a _secondary_ source (your link) that quotes from and does not cite a _primary_ source. All you can say with certainty is that this secondary source _says_ that primary source says what it does. The way you handle this in your document is to say something like "according to (secondary source), so-and-so said X in approximately (date)". You then cite your secondary source. You can increase the veracity of the claim about the primary source by finding other secondary sources that say the same thing. But if you haven't seen the primary source yourself, don't cite it. It will come across as dishonest, and if it turns out that your secondary source is _wrong_ about the primary source, it will call into question your credibility for anything _else_ you've cited.