avot patriarch fathers - Why do people say that Abraham and his family were idolaters at first?

A common idea these days, is that Abraham, his father and their families were supposed to have worshipped idols, until the true Creator, the LORD, called him from Ur of the Chaldees to give unto him and his seed the promised land (modern day Israel [and indeed much more...]).

Where does this idea come from? Is there proof of it in the Holy Scriptures?

It doesn't make sense to me, since other men which the LORD had 'called out' were all god-fearing men before the LORD called them for their specific purposes. For instance our grandfathers Noah, and Enoch, also kings Saul and David were all reverent to the LORD, and it appears that this is exactly the kind of man He would choose to use for such a great task as Abraham's.

A quote from the Judaism 101 website:

Abraham

According to Jewish tradition, Abraham was born under the name Abram in the city of Ur in Babylonia in the year 1948 from Creation (circa 1800 BCE). He was the son of Terach, an idol merchant, but from his early childhood, he questioned the faith of his father and sought the truth. He came to believe that the entire universe was the work of a single Creator, and he began to teach this belief to others.

Abram tried to convince his father, Terach, of the folly of idol worship. One day, when Abram was left alone to mind the store, he took a hammer and smashed all of the idols except the largest one. He placed the hammer in the hand of the largest idol. When his father returned and asked what happened, Abram said, "The idols got into a fight, and the big one smashed all the other ones." His father said, "Don't be ridiculous. These idols have no life or power. They can't do anything." Abram replied, "Then why do you worship them?"

Eventually, the one true Creator that Abram had worshipped called to him, and made him an offer: if Abram would leave his home and his family, then G-d would make him a great nation and bless him. Abram accepted this offer, and the b'rit (covenant) between G-d and the Jewish people was established. (Gen. 12).

Answer

Joshua 24:2

Thus said G-d, the L-rd of Israel: your ancestors lived on the other side of the [Euphrates] River from time immemorial; Terach, the father of Abraham and father of Nachor, and they worshipped foreign gods

It's a fascinating question (discussed by the classical commentaries in Genesis) exactly when/where humanity took a wrong turn, especially in the chain between Noah and Abraham, but clearly somewhere in the ancestral line there, there were idol worshippers.

(In context: Joshua is warning the people by making it clear that idol worship is in the people's DNA, and if they're not cautious, they'll find themselves sliding back into it -- which is exactly what happened after Joshua died.)