Al-Kāfi

Hadith 2322
etiquette
2ـ ابْنُ مَحْبُوبٍ عَنْ عَبْدِ الله بْنِ سِنَانٍ عَنْ أَبِي عَبْدِ الله (عَلَيهِ السَّلام) قَالَ رَدُّ جَوَابِ الْكِتَابِ وَاجِبٌ كَوُجُوبِ رَدِّ السَّلامِ وَالْبَادِي بِالسَّلامِ أَوْلَى بِالله وَرَسُولِهِ.
2. Ibn Mahbub has narrated from ‘Abd Allah ibn Sinan from Abu ‘Abdillah (a.s.) who has said the following: “It is necessary to reply a letter just as it is necessary to answer a greeting of peace. The initiator of a greeting of peace is closer to Allah and His messenger.”
Source: Shaykh Muḥammad b. Yaʿqūb al-Kulaynī