And Joshua said unto all the people, Thus saith the LORD God of Israel, Your fathers dwelt on the other side of the flood in old time, even Terah, the father of Abraham, and the father of Nachor: and they served other gods. On the other side of the flood - The river Euphrates.
They served other gods - Probably Abraham as well as Terah his father was an idolater, till he received the call of God to leave that land. See on Genesis 11:31 (note); Genesis 12:1 (note).