'Abdullah ibn as-Samit said, "I said to Abu Dharr, 'I passed by 'Abdu'r-Rahman ibn Umm al-Hakam and I greeted him and he did not answer me at all. He said, 'Nephew, what harm have suffered from that? Someone better than him answered you ? the angel on his right."  

عَنْ يَمِينِهِ  

'Abdullah said, "Peace is one of the Names of Allah which Allah has placed on the earth. Extend it among you. When a man greets people and they answer him, then he has a higher degree than them because he reminds them of peace. If no one answers him, he will be answered by one who is better and more excellent.'"  

عَنْ عَبْدِ اللهِ قَالَ إِنَّ السَّلاَمَ اسْمٌ مِنْ أَسْمَاءِ اللهِ وَضَعَهُ اللَّهُ فِي الأَرْضِ فَأَفْشُوهُ بَيْنَكُمْ إِنَّ الرَّجُلَ إِذَا سَلَّمَ عَلَى الْقَوْمِ فَرَدُّوا عَلَيْهِ كَانَتْ لَهُ عَلَيْهِمْ فَضْلُ دَرَجَةٍ لأَنَّهُ ذَكَّرَهُمُ السَّلاَمَ وَإِنْ لَمْ يُرَدَّ عَلَيْهِ رَدَّ عَلَيْهِ مَنْ هُوَ خَيْرٌ مِنْهُ وَأَطْيَبُ  

Al-Hasan said, "Greeting is an act of obedience while the answer is a duty."  

عَنِ الْحَسَنِ قَالَ التَّسْلِيمُ تَطَوَّعٌ وَالرَّدُّ فَرِيضَةٌ