I haven't said that that "Love God above all and neighbor as self is the greatest law of all" is written in the Koran. I did not explain it well. What I'm trying to say is that those who worship the same God in spirit and truth follow this same precept as the eternal moral law because is the SAME GOD we worship. This same God has told us this in the Bible, He may have not told with the same wording in the Koran, but this doesn't mean that He has changed His mind for Muslims. Since the one true God of all of humanity is Love, He can not be so just for Christians, hate for Muslim or immorality for Jewish, etc. God is Love and His love applies to all of us regardless of the religion people follow.
Actually, it doesn't have to be written in the Koran, for it's already "written" in our hearts. For example, when Cain killed Abel, Cain knew that murdering his brother was wrong and the Ten Commandments were not written yet, and because of this reason, it doesn't mean that Cain was exempted from his fault. So, for us reasonable humans, we know what an immorality is, even when nobody is watching or nobody has told us that it is wrong.
Here is a basic information from Wickipedia explaining how the Muslin seeking peace and love interpret the Koran, and those who don't:
Now, any one who, for example takes that particular passage from the Koran 8:12, and takes it 'by the letter' and do it "because it's written there", at that specific point, and does not take other considerations (also written in the Koran) and of even other Muslims seeking peace, then he is an idolater, not a true worshiper of God.