This is not about who does the Golan belong to its about Israel warning Assad not to let to let the fire from the Syrian civil war spread into the territory that under the terms of Resolution 242 is in Israeli hands until such time its Israel & Syria reach an agreement over its future in exchange for a JUST AND LASTING PEACE. As this is something we don't have right now then the Golan is in Israeli hands. This warning is being given to Assad because if it continues then to protect its population then Israel may be forced to intervene which is going to turn a Syrian civil war into an arab / Israel war. In this case this is Syria firing on Israeli citizens so Assad is initiating the violence and Israel's warning a shot across the bows to avoid an escalation which will involve Israel so its an attempt to avoid a war.
High school teacher's union threatens strike (Haaretz)
from the article: Israel warns Assad over stray Syrian fire on Golan Heights