عَنْ هِشَامِ بْنِ زَيْدِ بْنِ أَنَسٍ رَضِيَ اللَّهُ عَنْهُ قَالَ سَمِعْتُ أَنَسًا رَضِيَ اللَّهُ عَنْهُ يَقُولُ جَاءَ رَجُلٌ مِنْ أَهْلِ الْكِتَابِ فَسَلَّمَ عَلَى النَّبِيِّ صَلَّى اللَّهُ عَلَيْهِ وَآلِهِ وَسَلَّمَ فَقَالَ السَّامُ عَلَيْكُمْ فَقَالَ عُمَرُ رَضِيَ اللَّهُ عَنْهُ يَا رَسُولَ اللَّهِ أَلَا أَضْرِبُ عُنُقَهُ قَالَ لَا إِذَا سَلَّمُوا عَلَيْكُمْ فَقُولُوا وَعَلَيْكُمْ
It is narrated from Sayyiduna Anas (may Allah be pleased with him), he says: A man from among the People of the Book came and greeted the Noble Prophet (peace and blessings be upon him) saying: "As-Saam ‘alaykum" (death be upon you). Sayyiduna ‘Umar (may Allah be pleased with him) said: O Messenger of Allah! Shall I not strike off his neck? The Prophet (peace and blessings be upon him) said: No, just when these people greet you, respond to them by saying: "Wa ‘alaykum" (and upon you).