by using folloing command:
yumdownloader --source 'package-name-without-version'
It will only download the newest source rpm of said software.
how do I download specified version of source rpm using yumdownloader?
You can do that using
yumdownloader --source <package>-<version>
For example,
yumdownloader --source httpd-2.4.6-89.el7.centos