The Hague Convention
18 October 1907

  Art. 23. In addition to the prohibitions provided by special
  conventions, it is especially forbidden - (c) To kill or wound an
  enemy who having laid down his arms, or having no longer
  means of defence, has surrendered at discretion.
  


The Lieber Code
24 April 1863

  Art. 49. All soldiers, of whatever species of arms; all men who
  belong to the rising en masse of the hostile country; all those who
  are attached to the army for its efficiency and promote directly the
  object of the war, except such as are hereinafter provided for; all
  disabled men or officers on the field or elsewhere, if captured; all
  enemies who have thrown away their arms and ask for quarter,
  are prisoners of war, and as such exposed to the inconveniences
  as well as entitled to the privileges of a prisoner of war.
  


The Geneva Conventions
12th of August 1949

  Art. 3. (1) Persons taking no active part in the offences,
  including members of armed forces who have laid down their
  arms and those placed hors de combat by sickness, wounds,
  detention, or any other cause, shall in all circumstances be
  treated humanely, without any adverse distinction founded on
  race, colour, religion or faith, sex, birth or wealth, or any
  other similar criteria.