It refers to the C++ standard, also known as "ISO/IEC 14882", an official document published by ISO that specifies the syntax and semantics of the standard C++ language.
Unfortunately the standard is actually more expensive than most books, but you can find drafts online that are close to the official standards. See: Where do I find the current C or C++ standard documents?
So far there have been four standards, published in 1998, 2003, 2011, and 2014, hence the names C++98, C++03, C++11, and C++14. The OP of the other question didn't specify which revision they're referring to. I checked both the 2011 and 2014 standards, and they both have the given quote at the given paragraph number.
The numerical reference, "27.7.3.6.2/1", may change from one revision to the next. "[ostream.inserters.arithmetic]/1" is more stable, since the section names don't change, but paragraph numbers and wording might still change, so it's important to specify which revision you're referring to.