(475) بَابُ مَنْ لَمْ يَرُدَّ السَّلامَ
Al-Hasan said, "Greeting is an act of obedience while the answeris a duty."
Reference: Al-Adab Al-Mufrad 1040
In-book reference: Book 42, Hadith 77
English translation: Book 42, Hadith 1040