Comment by Jtsummers

Erlang, Elixir, Ada, plenty of others. Erlang and Ada predate Go by several decades, too.

You wanted sources, here's the chapter on tasks and synchronization in the Ada LRM: http://www.ada-auth.org/standards/22rm/html/RM-9.html

For Erlang and Elixir, concurrent programming is pretty much their thing so grab any book or tutorial on them and you'll be introduced to how they handle it.