Only NTP sources can be specified in the /etc/chrony/sources.d directory.
Files in this directory must end with the ".sources" suffix. They can only
contain the "peer", "pool" and "server" directives and must have all
lines terminated by a trailing newline.

There is no need to restart chronyd for these time sources to be usable,
running 'chronyc reload sources' is sufficient.

Example:

# echo 'server 192.0.2.1 iburst' > /etc/chrony/sources.d/local-ntp-server.sources
# chronyc reload sources