By what authority did Lincoln issue the Emancipation Proclamation?

Executive power of "necessary and proper"--Lincoln was able to legislate from the Oval by use of executive order and in this case as Commander in Chief of the army.

Lincoln used the Emancipation Proclamation as a means to control the message of the Civil War, boost morale, and target the Southern labor force. As Commander and Chief, he has able to make war decisions and as it was an internal rebellion all acts were done by his executive order.