If we can rely on Wikipedia:
It's the Hague (not Geneva which came later) Convention of 1899.
"Expanding bullets, also known colloquially as dumdum bullets, are projectiles designed to expand on impact. This causes the bullet to increase in diameter, to combat over-penetration and produce a larger wound. This leads to a living target being incapacitated faster. For this reason they are used for hunting and by some police departments, but are generally prohibited for use in war. Two typical designs are the hollow-point bullet and the soft-point bullet."
"International law
The Hague Convention of 1899, Declaration III prohibits the use of expanding bullets in international warfare.[21][22] This is often incorrectly believed to be prohibited by the Geneva Conventions, but it significantly predates those conventions, and is in fact a continuance of the Declaration of St Petersburg in 1868, which banned exploding projectiles of less than 400 grams.
The text of the declaration states, "The present Declaration is only binding for the Contracting Powers in the case of a war between two or more of them".[21] Until relatively recently, the prohibition on the use of expanding bullets was applicable only to international armed conflicts between the countries that have signed it. According to the International Committee of the Red Cross's customary international law study, customary international law now prohibits their use in any armed conflicts.[22][23] This has been disputed by the United States, which maintains that the use of expanding bullets can be legal when there is a clear military necessity.[23] The adoption of an amendment to Article 8 at the Review Conference of the Rome Statute in Kampala makes the use of expanding bullets in non-international armed conflict a war crime.[22][24] One example of a war crime involving expanding ammunition is the August 1941 German killing of Soviet prisoners at Zhitomir, as a human experiment with captured Red Army materiel.[25]
Because the Hague Convention applies only to the use of expanding bullets in war, the use of expanding rounds remains legal in other circumstances unless it is restricted or prohibited by local laws. Examples are use of appropriately expanding bullets in hunting in which it is desirable to stop the animal quickly, either to prevent loss of a game animal or to ensure a humane death of the animal, and in law enforcement or self-defence if quickly neutralising an aggressor may be needed to prevent further loss of life or the bullet must remain inside the target to prevent collateral damage."
https://en.wikipedia.org/wiki/Expanding_bullet