Abdullah ibn San'an narrated that Imam Sadiq (a.s) said
“It is obligatory to reply to letters as it is to reply to greetings.”
عن عبد الله بن سنان عن أبي عبد الله (عليه السلام) قال
رد جواب الكتاب واجب كوجوب رد السلام.
Similar narrations
Al-Kāfi #2322“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...