According to Genesis, Abraham hailed from a Mesopotamian city called “Ur Kasdim,” a name usually rendered in English as “Ur of the Chaldeans.” Most commentators have read this passage to mean that Abraham was born in Ur, although the great 13th-century rabbinic scholar and exegete Moses Nahmanides argues that Abraham was born not in Ur but in the land of Haran, in northern Mesopotamia. Reuven Chaim Klein disposes of this claim in light of rabbinic commentaries and archaeological knowledge about the city of Ur and the Chaldean language:
According to [traditional Jewish] versions of the narrative, Abraham’s family escaped Ur and relocated to Aram [i.e., the northern part of Mesopotamia] in order to flee from the influence of Nimrod. The reason for their escape is recorded by tradition: Nimrod—civilization’s biggest sponsor of idolatry—sentenced Abraham to death by fiery furnace for his iconoclastic stance against idolatry. After Abraham miraculously emerged unscathed from the inferno, his father Terah decided to relocate the family from Ur (within Nimrod’s domain) to the city of Haran in the Aram region, which was relatively free from Nimrod’s reign of terror (Gen. 11:31). It was from Haran that Abraham later embarked on his historic journey to the land of Canaan (Gen. 12).
Josephus in Antiquities of the Jews mentions a similar version of events. He quotes the first-century Greek historian Nicolaus of Damascus who wrote that Abraham, a “foreigner” from Babylonia, came to Aram. There, he reigned as a king for some time, until he and his people migrated to the land of Canaan.
Read more on Seforim: http://seforim.blogspot.com/2014/12/abrahams-chaldean-origins-and-chaldee.html