Al-Adab Al-Mufrad, Book 42, Hadith 1040 (English) ⓘ
Al-Hasan said, "Greeting is an act of obedience while the answer is a duty."
حَدَّثَنَا مُحَمَّدُ بْنُ يُوسُفَ، قَالَ: حَدَّثَنَا سُفْيَانُ، عَنْ هِشَامٍ، عَنِ الْحَسَنِ قَالَ: التَّسْلِيمُ تَطَوَّعٌ، وَالرَّدُّ فَرِيضَةٌ.
also referenced under: Hadith 1040 (Reference) & Book 42, Hadith 77 (In-Book)