Deuteronomy 26:12 Every third year you must offer a special tithe of your crops. In this year of the special tithe you must give your tithes to the Levites, foreigners, orphans, and widows, so that they will have enough to eat in your towns.
The person who delivered the tithe was then to recite a dedication to God, and it ended with:
Deuteronomy 26:15 Now look down from your holy dwelling place in heaven and bless your people Israel and the land you swore to our ancestors to give us …
This concept of blessings (for obedience) and curses (for disobedience) was further developed.
It can be summarised here:
Deuteronomy 26:18-19 The Lord has declared today that you are his people, his own special treasure, just as he promised, and that you must obey all his commands. And if you do, he will set you high above all the other nations he has made. Then you will receive praise, honor, and renown. You will be a nation that is holy to the Lord your God, just as he promised.