From CojsWiki
An extra-biblical reference to the observance of the Sabbath
According to the Bible, the origin of the Sabbath is the creation of the world:
“And God blessed the seventh day and declared it holy, because on it God ceased from all the work of creation that He had done.” (Genesis 2:3)
The Bible proscribes observance of the Sabbath:
“Remember the Sabbath day and keep it holy.” (Exodus 20:8)
The observance of the Sabbath is reported:
"And the Children of Israel observed the Sabbath, to make the Sabbath for their generations an eternal covenant. Between Me and the Children of Israel it is a sign forever..." (Exodus 31:16)
The text of the Sabbath Ostraca is as follows:
Greetings, Yislah!
Look, I'm sending you the vegetables tomorrow. Get there before the boat comes in - on account of the the Sabbath - so they won't be spoiled. I swear to God, if you don't, I'll kill you! Don't trust Meshullemet or Shemayah to take care of it.
Trade the barley for me. Try to get a lamb... I swear to God, if you don't, you'll have to... yourself.
As for the... you sent in exchange for the wine, I have sent it all on to them (?). If Meshullemet doesn't get here, what will you say?
May we see each other soon!