You can combine this with Nixhub to find the precise commit of nixpkgs, if you need a copy of stable for a specific package! For example, I needed to get draw.io before it changed its license to a non-FOSS, so I pinned it to a specific commit hash.

The commit hash goes in the configuration like this (example): nixpkgs-lvm.url = "github:nixos/nixpkgs/2fbfb1d73d239d2402a8fe03963e37aab15abe8b";