Hadith on Salaam: Greeting unbelievers with peace

Abu Uthman reported: Abu Musa, may Allah be pleased with him, wrote to a Persian leader and greeted him with peace in his letter. It was said, “Do you greet him with peace when he is an unbeliever?” Abu Musa said, “Indeed, he wrote to me and greeted me with peace, so I have returned it.”

Source: al-Adab al-Mufrad 1101

Grade: Sahih (authentic) according to Al-Albani

عَنْ أَبِي عُثْمَانَ النَّهْدِيِّ قَالَ‏ كَتَبَ أَبُو مُوسَى رضي الله عنه إِلَى رُهْبَانٍ يُسَلِّمُ عَلَيْهِ فِي كِتَابِهِ فَقِيلَ لَهُ‏ أَتُسَلِّمُ عَلَيْهِ وَهُوَ كَافِرٌ‏ قَالَ‏ إِنَّهُ كَتَبَ إِلَيَّ فَسَلَّمَ عَلَيَّ فَرَدَدْتُ عَلَيْهِ

1101 الأدب المفرد للبخاري باب إذا كتب الذمي فسلم

837 المحدث الألباني خلاصة حكم المحدث صحيح في صحيح الأدب المفرد