A Jew passed by Messenger of Allah ﷺ and said, "As-Samu 'Alaika." Messenger of Allah ﷺ said in reply, "We 'Alaika." Messenger of Allah ﷺ then said to his companions, "Do you know what he (the Jew) has said? He said, 'As-Samu 'Alaika.'" They said, "O Messenger of Allah ﷺ! Shall we kill him?" The Prophet, said, "No. When the people of the Book greet you, say: 'Wa 'Alaikum.'"  

سَمِعْتُ أَنَسَ بْنَ مَالِكٍ يَقُولُ مَرَّ يَهُودِيٌّ بِرَسُولِ اللَّهِ ﷺ فَقَالَ السَّامُ عَلَيْكَ فَقَالَ رَسُولُ اللَّهِ ﷺ وَعَلَيْكَ فَقَالَ رَسُولُ اللَّهِ ﷺ أَتَدْرُونَ مَا يَقُولُ قَالَ السَّامُ عَلَيْكَ قَالُوا يَا رَسُولَ اللَّهِ أَلاَ نَقْتُلُهُ قَالَ لاَ إِذَا سَلَّمَ عَلَيْكُمْ أَهْلُ الْكِتَابِ فَقُولُوا وَعَلَيْكُمْ  

A group of Jews asked permission to visit the Prophet (and when they were admitted) they said, "As- Samu 'Alaika (Death be upon you)." I said (to them), "But death and the curse of Allah be upon you!" The Prophet ﷺ said, "O ʿAisha! Allah is kind and lenient and likes that one should be kind and lenient in all matters." I said, "Haven't you heard what they said?" He said, "I said (to them), 'Wa 'Alaikum (and upon you).  

قَالَتِ اسْتَأْذَنَ رَهْطٌ مِنَ الْيَهُودِ عَلَى النَّبِيِّ ﷺ فَقَالُوا السَّامُ عَلَيْكَ فَقُلْتُ بَلْ عَلَيْكُمُ السَّامُ وَاللَّعْنَةُ فَقَالَ يَا عَائِشَةُ إِنَّ اللَّهَ رَفِيقٌ يُحِبُّ الرِّفْقَ فِي الأَمْرِ كُلِّهِ قُلْتُ أَوَلَمْ تَسْمَعْ مَا قَالُوا قَالَ قُلْتُ وَعَلَيْكُمْ  

Messenger of Allah ﷺ said, "When the Jews greet anyone of you they say: 'Sam'Alaika (death be upon you); so you should say; 'Wa 'Alaika (and upon you).'"  

قَالَ رَسُولُ اللَّهِ ﷺ إِنَّ الْيَهُودَ إِذَا سَلَّمُوا عَلَى أَحَدِكُمْ إِنَّمَا يَقُولُونَ سَامٌ عَلَيْكَ فَقُلْ عَلَيْكَ