Sayyiduna Ibn Abbas (may Allah be pleased with him) narrates from the Prophet (peace and blessings be upon him) that he said: "Indeed, replying to a letter is as necessary as replying to a greeting (salam)." The Shaykh says: Its chain of narration is not strong.
Hadith Referenceمسند الشهاب / 1010
Hadith Gradingمحدثین:إسناده ضعیف
Hadith Takhrijإسناده ضعیف ، شریک بن عبداللہ نخعی مدلس و مختلط ہے ، اس میں ایک اور بھی علت ہے ۔