Are you running chrony or ntpd?
Check /etc/ntp.conf or /etc/chrony/chrony.conf. Also check in /etc/chrony/chrony.conf.d because files in there are automatically included in chronyd's list of sources by default.
If you're running systemd-timesyncd, then you'll have to check its default config files, and I've really no idea where they live. I disable/uninstall that and install chrony on all the machines I manage at work or at home.