Abu Uthman an-Nahdi said: Abu Musa wrote to a Persian leader and greeted him with peace in his letter. He was asked, “Do you greet him with peace when he is a disbeliever?” He replied, “Indeed, he wrote to me and greeted me with peace, so I have answered him.”

[Bukhari, Al-Adab Al-Mufrad #1101]

عَنْ أَبِي عُثْمَانَ النَّهْدِيِّ قَالَ‏:‏ كَتَبَ أَبُو مُوسَى إِلَى رُهْبَانٍ يُسَلِّمُ عَلَيْهِ فِي كِتَابِهِ، فَقِيلَ لَهُ‏:‏ أَتُسَلِّمُ عَلَيْهِ وَهُوَ كَافِرٌ‏؟‏ قَالَ‏:‏ إِنَّهُ