حَدَّثَنَا يَحْيَى بْنُ بِشْرٍ، قَالَ: حَدَّثَنَا الْحَكَمُ بْنُ الْمُبَارَكِ، قَالَ: حَدَّثَنَا عَبَّادٌ يَعْنِي ابْنَ عَبَّادٍ، عَنْ عَاصِمٍ الأَحْوَلِ، عَنْ أَبِي عُثْمَانَ النَّهْدِيِّ قَالَ: كَتَبَ أَبُو مُوسَى إِلَى رُهْبَانٍ يُسَلِّمُ عَلَيْهِ فِي كِتَابِهِ، فَقِيلَ لَهُ: أَتُسَلِّمُ عَلَيْهِ وَهُوَ كَافِرٌ؟ قَالَ: إِنَّهُ كَتَبَ إِلَيَّ فَسَلَّمَ عَلَيَّ، فَرَدَدْتُ عَلَيْهِ.
It is narrated from Abu Uthman Nahdi (may Allah have mercy on him) that Sayyiduna Abu Musa Ash'ari (may Allah be pleased with him) wrote a letter to a non-Muslim chief (or monk) and included greetings (salam) in it. He was asked: "You greet him even though he is a disbeliever?" He replied: "He had written greetings to me in his letter, so I replied to him."