Create backup from /etc/apt/source.list
As /etc/apt/sources.list is just a file, you could create a "backup" by copying the file to something with a different name:
sudo cp /etc/apt/sources.list /etc/apt/sources.orig
This will create a new file in /etc/apt called sources.orig with the very same contents as sources.list. sudo is needed because the file is owned by the root account.