When Jacob fled from his brother Esau and set out on the long road to Haran, he stopped at a place called Bethel and made a vow to God. "If God will be with me and guard me on this journey," Jacob said, "and give me bread to eat and clothing to wear, and I return safely to my father's house — then the Lord shall be my God, and of everything You give me, I will surely give a tenth to You" (Genesis 28:20-22).
This vow, the Midrash (rabbinic interpretive commentary) teaches, established the principle of tithing — giving one-tenth of one's earnings to God — for all future generations of Israel. The Pesikta (f. 98a) and Tanchuma (Deut., Reeh) explore why Jacob chose a tenth rather than a fifth or a third.
The rabbis explained that Jacob was not being stingy. He was being precise. A tenth is the exact amount that acknowledges God's ownership of everything without crushing the giver. Too little, and the gift is an insult. Too much, and the giver impoverishes himself and becomes a burden on others. The Talmud would later rule that a person should not give more than a fifth of their wealth to charity, lest they themselves need charity.
Bereshit Rabbah (70:7) adds that Jacob kept his vow scrupulously. Of his twelve sons, he dedicated one-tenth — Levi — to the service of God. The tribe of Levi received no land inheritance in Israel, because they belonged entirely to God. Jacob's midnight vow at Bethel became the foundation of an entire tribe's destiny.