Zaerih asked Imam Sadiq (a.s) about greeting the Jews and the Christians, and responding to their letters. Imam Sadiq (a.s) said that it is not recommended to respond to their letters.
عن ذريح قال: سألت أبا عبد الله (عليه السلام) عن التسليم على اليهودي والنصراني والرد عليهم في الكتاب، فكره ذلك.
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Mishkāt al-Anwār #741Zarih narrated that when he asked Imam Sadiq (a.s) about greeting the Christians and the Jews, and replying to their letters, the Imam was...