The State of Nagaland Act 1962
Section 3. Formation of State of Nagaland
(1) As from the appointed day, there shall be formed a new State to be known as the State of Nagaland comprising the territories which immediately before that day were comprised in the Naga Hills-Tuensang Area and thereupon the said territories shall cease to form part of the State of Assam.
(2) Without prejudice to the power of the State Government to alter after the appointment day, the name, extent or boundaries of any district, the State of Nagaland shall consist of three districts to be called the Kohima district, Mokokchung district and Tuensang district, each comprising the areas respectively set out in the Schedule.