Which regulation does this sentence about IPv6 refer to?

"For this, you need to obtain an IPv6 address from your ISP. Technically this could be a /128 prefix (exactly one IPv6 address), but according to regulation it needs to be at least a /64 prefix."

from the OpenWRT Wiki

To what regulations are they referring? (So that if I need to defend myself when the time comes, I'll be able to point to something)

It likely refers to the RFC 6177.

  A key principle for address management is that end sites always be
  able to obtain a reasonable amount of address space for their
  actual and planned usage, and over time ranges specified in years
  rather than just months.  In practice, that means at least one
  /64, and in most cases significantly more.  One particular
  situation that must be avoided is having an end site feel
  compelled to use IPv6-to-IPv6 Network Address Translation or other
  burdensome address conservation techniques because it could not
  get sufficient address space.