Annex to the Convention: Regulations respecting the laws and customs of war on land - Section II : Hostilities - Chapter V : Armistices - Regulations: Art. 36.
Art. 36. An armistice suspends military operations by mutual agreement between the belligerent parties. If its duration is not defined, the belligerent parties may resume operations at any time, provided always that the enemy is warned within the time agreed upon, in accordance with the terms of the armistice.