What Is the Earliest Source? Intro and 1

We've had lively discussions on various lists re: the antiquity [or lack thereof] of various Halachot that we now take for granted.

Sometimes I can readily identify the earliest source - and sometimes I can get there with just a bit of research.

And for some, I do not even know the earliest source myself.
And perhaps readers can supply one themselves - thereby saving me the hard work of doing my own research!

I have a long list but in order to stimulate more activity on our blog - I will put them out one at a time.
Actually that is only part of the reason, the main point is to organize discreet posts in order to make research and the comments more focused.

Here goes the first item:

1. Where may be found the earliest source specifying a brachah on Ner Shabbat?

KT
RRW