Quotation for Gittin 120:1

This is why the Rabbis say, God made a covenant with Israel only for the sake of the oral law. This is because the written law can not be understood except with the oral law, and also because the law of God can not be perfect so as to be adequate for all times, because the ever new details of human relations, their customs and their acts, are too numerous to be embraced in a book. Therefore Moses was given orally certain general principles, only briefly alluded to in the Torah, by means of which the wise men in every generation may work out the details as they appear.