In the Bible the "sabbath" is the seventh-day rest commanded in the Ten Commandments and rooted in Genesis, when God rested on the seventh day of creation.
Exodus 20:8-11 in the King James Version reads:
8 Remember the sabbath day, to keep it holy. 9 Six days shalt thou labour, and do all thy work: 10 But the seventh day is the sabbath of the LORD thy God: in it thou shalt not do any work, thou, nor thy son, nor thy daughter, thy manservant, nor thy maidservant, nor thy cattle, nor thy stranger that is within thy gates: 11 For in six days the LORD made heaven and earth, the sea, and all that in them is, and rested the seventh day: wherefore the LORD blessed the sabbath day, and hallowed it.
The commandment appears in Exodus 20, as the fourth of the Ten Commandments given at Mount Sinai. Read it in context at Exodus 20:8-11, or the full chapter at Exodus 20. Encyclopedia entry: en.wikipedia.org/wiki/Biblical_Sabbath.