It is narrated from Sayyiduna Ibn Abbas (may Allah be pleased with them) that he said: I consider replying to a letter as necessary as replying to a greeting of peace (salam).
Hadith Referenceالادب المفرد / كتاب الرسائل / 1117
Hadith Gradingالألبانی:حسن
Hadith Takhrij«حسن : أخرجه ابن أبى شيبة : 26369 و ابن الجعد : 2399 و البيهقي فى شعب الإيمان : 9097»