Many people find it easier to handle backing up and restoring a single file rather than dealing with a directory of files (as the other mentioned solutions require). If you are like this, and you do not care about having each PPA stored in its own file inside of /etc/apt/sources.list.d/, you can use the following command to store all of your added repositories in a single file called sources.list located in your home directory.
cat /etc/apt/sources.list /etc/apt/sources.list.d/* > ~/sources.list
You could then move this file to /etc/apt/sources.list and do sudo apt-get update to re-add the repositories. If you are planning to use this backup on another computer, make sure that the version of Ubuntu on the machine matches the versions in the sources.list file, otherwise, you might have some problems.