I don't know where Hamas could even find medium-range SAM but Assad, as much as he doesn't like Hamas, would like to take down several israeli planes so he could have proliferated them a SAM or two, nothing is impossible today.
I'd imagine the Israelis keep a very beady eye on anything that looks like a launcher moving through Syria towards Lebanon. In 1982 they hammered Syrian SAM batteries in the region and even recently convoys have been destroyed in that area that were allegedly intended for Hamas.