Israeli political and military affairs have repeatedly addressed how to respond in the event that Iran attacks Israel. According to politicians, if Iran targets residential centers inside Israel, even if it comes from Syria or with the help of Hezbollah or other groups, the retaliation will be directed to Iranian territory.
Neither the Hezbollah nor the Assad's army is stupid enough to fire missiles towards Tel Aviv or Jerusalem. My biggest fear is that some American funded militant group in Syria will do this, and the Americans and the Israelis will blame the attack on Iran. Just like they blamed the recent chemical attack on Assad's forces.