The first Rashi on the Torah asks and answers a famous question:
Why does the Torah, meant primarily as a means to derive Jewish laws and not meant to be a history book (the word Torah actually means "law,") start with "In the beginning of God's creation..."? Why doesn't it start with the first commandment given to Jews, about the sanctifying of new months?
The answer is based on Psalms 111:6, "He declared to His people the power of His works, in giving them the heritage of the nations."
For if the nations of the world should say to Israel, “You are robbers, for you conquered by force the lands of the seven nations [of Canaan],” they will reply, "The entire earth belongs to the Holy One, blessed be He; He created it (this we learn from the story of the Creation) and gave it to whomever He deemed proper When He wished, He gave it to them, and when He wished, He took it away from them and gave it to us.
But beyond that, the Jewish sages taught, there are three places that are indisputably owned by Jews, because they were purchased even before God fulfilled his promise to the forefathers. The three places are the Cave of the Patriarchs in Hebron purchased by Abraham, the Temple Mount purchased by King David, and a section of Shechem (Nablus) purchased by Jacob from the family of the Canaanite ruler Hamor.
I will not be discussing the irony of how so many tenaciously view these very places as places that Jews specifically have no rights to.
I saw something interesting about Abraham's purchase in Hebron. How large was the land he bought?
All we know is that he paid 400 silver shekels. (Gen. 23:16)
Rabbi Yitzchak bar Yehudah (13th century CE) notes that in Leviticus 27:16, it says that the value of a field sized where you could plant a "chomer" of barley seeds is fifty silver shekels.
A chomer is also known as a "kor" and it is equal to 30 se'ah in volume.
The Talmud (Eruvin 96a) defines the size of a field that could be sown with a se'ah to be 2500 square cubits (Two "beit se'ah" is equal to 5000 square amot/cubits, the size of the Temple courtyard.)
So the size of the 50-shekel measure mentioned in Leviticus, also known as a "beit kor", is 75,000 square amot.
Abraham paid 400 shekels, so at the going rate he paid for 600,000 square cubits of land.
The Talmud says that a man standing up takes up one square cubit. 600,000 is the number that is used to signify the total number of Jews, as it is roughly the number of Jews counted that left Egypt.
This means that Abraham's purchase corresponds to the number of Jews counted when the Children of Israel became a nation.
In other words, when Abraham made the first purchase of land in Israel, he bought enough to assure that every Jewish soul has a place in the Land of Israel!
(h/t Chabad)