Numbers 18:12-18, 27-30

All the best of the oil and all the best of the wine and of the grain, the firstfruits of what they give to the LORD, I give to you. The first ripe fruits of all that is in their land, which they bring to the LORD, shall be yours. Everyone who is clean in your house may eat it. Every devoted thing in Israel shall be yours. Everything that opens the womb of all flesh, whether man or beast, which they offer to the LORD, shall be yours. Nevertheless, the firstborn of man you shall redeem, and the firstborn of unclean animals you shall redeem. And their redemption price (at a month old you shall redeem them) you shall fix at five shekels in silver, according to the shekel of the sanctuary, which is twenty gerahs. But the firstborn of a cow, or the firstborn of a sheep, or the firstborn of a goat, you shall not redeem; they are holy. You shall sprinkle their blood on the altar and shall burn their fat as a food offering, with a pleasing aroma to the LORD. But their flesh shall be yours, as the breast that is waved and as the right thigh are yours.

And your contribution shall be counted to you as though it were the grain of the threshing floor, and as the fullness of the winepress. So you shall also present a contribution to the LORD from all your tithes, which you receive from the people of Israel. And from it you shall give the LORD’s contribution to Aaron the priest. Out of all the gifts to you, you shall present every contribution due to the LORD; from each its best part is to be dedicated.’ Therefore you shall say to them, ‘When you have offered from it the best of it, then the rest shall be counted to the Levites as produce of the threshing floor, and as produce of the winepress.