That the Messenger of Allah ﷺ said: "Indeed when a Jew gives Salam to one of you, then he is only saying: 'As-Samu 'Alaikum' (Death be upon you) so say: "Alaik (And upon you).'" (Using translation from Tirmidhī 1603)  

قَالَ رَسُولُ اللَّهِ ﷺ إِنَّ الْيَهُودَ إِذَا سَلَّمَ أَحَدُهُمْ فَإِنَّمَا يَقُولُ السَّامُ عَلَيْكَ قُلْ عَلَيْكَ