Hi,
you'll need to have the dnf-plugins-core package installed to have access to the "download" command for dnf.
Are you sure ?
I tried this sequence on Fedora 22:
$ dnf download --source iproute enabling fedora-source repository enabling updates-source repository Last metadata expiration check performed 0:01:35 ago on Tue Aug 25 08:04:37 2015. iproute-3.16.0-3.fc22.src.rpm 689 kB/s | 615 kB 00:00
$ rpm -q dnf-plugins-core package dnf-plugins-core is not installed
Regards, Rami Rosen http://ramirose.wix.com/ramirosen