Abu 'Uthman an-Nahdi said, "Abu Musa wrote to a Persian grandee and greeted him in his letter. He was asked, 'Do you greet him when he is an unbeliever?' He replied, 'He wrote to me and greeted me, so I have answered him.'"  

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