Long before the laws of Moses were given, tithing was practiced by Abraham. Abraham gave a tenth of his increase to Melchizedek, the High Priest of Salem.

Hebrews 7:1-5  For this Melchisedec, king of Salem, priest of the most high God, who met Abraham returning from the slaughter of the kings, and blessed him;  To whom also Abraham gave a tenth part of all; first being by interpretation King of righteousness, and after that also King of Salem, which is, King of peace;  Without father, without mother, without descent, having neither beginning of days, nor end of life; but made like to the Son of God; abideth a priest continually.  Now consider how great this man was, to whom even the patriarch Abraham gave the tenth of the spoils. 

This is also mentioned in Genesis 14:17-20 

And the king of Sodom went out to meet him after his return from the slaughter of Chedorlaomer, and of the kings that were with him, at the valley of Shaveh, which is the king's dale.  And Melchizedek king of Salem brought forth bread and wine: and he was the priest of the most high God.  And he blessed him, and said, Blessed be Abram of the most high God, possessor of heaven and earth:  And blessed be the most high God, which hath delivered thine enemies into thy hand. And he gave him tithes of all.