> # apt-get source openssl
> Reading package lists... Done
> E: You must put some 'source' URIs in your sources.list

Have you run apt-get update after having modified sources.list?

> # cat /etc/apt/sources.list
> ## package repositories
> deb http://deb.devuan.org/merged beowulf main
> deb http://deb.devuan.org/merged beowulf-updates main
> deb http://deb.devuan.org/merged beowulf-security main
> deb http://deb.devuan.org/merged beowulf-backports main
>
> ## source repositories
> deb-src http://deb.devuan.org/merged beowulf main
> deb-src http://deb.devuan.org/merged beowulf-updates main
> deb-src http://deb.devuan.org/merged beowulf-security main
> deb-src http://deb.devuan.org/merged beowulf-backports main

Your sources are correct, even overkill as pointed out by others.
For the openssl sources, you only require:
deb-src http://deb.devuan.org/merged beowulf main

Bernard (Beer) Rosset
https://rosset.net/