It is narrated from Hasan al-Basri, may Allah have mercy on him, that he said: Saying salam is voluntary, and responding to it is obligatory.
Hadith Referenceالادب المفرد / كتاب السلام / 1040
Hadith Gradingالألبانی:صحيح
Hadith Takhrij«صحيح : الجامع الصغير للسيوطي ، حديث : 4848»
Explanation & Benefits
Shaykh Maulana Usman Muneeb
Benefits and Issues: This is the opinion of Hasan al-Basri rahimahullah; otherwise, upon reviewing all the evidences, it becomes clear that offering greetings of peace (salam) and responding to it are among the rights of Muslims, and the Lawgiver sallallahu alayhi wa sallam has commanded the fulfillment of these rights. Thus, both giving salam and responding to it are necessary.
Source: Fadlullah al-Ahad: Urdu Commentary on al-Adab al-Mufrad, Page: 1040