(sic) Abu'z-Zinad said, "Kharija (ibn Zayd ibn Thabit) used to write on the letter of Zayd when he said the salam, 'Peace be upon you, Amir al-Mu'minin, and the mercy of Allah and His blessings and His forgiveness and the best of His prayers."  

عَنْ أَبِي الزِّنَادِ قَالَ كَانَ خَارِجَةُ يَكْتُبُ عَلَى كِتَابِ زَيْدٍ إِذَا سَلَّمَ قَالَ السَّلاَمُ عَلَيْكَ يَا أَمِيرَ الْمُؤْمِنِينَ وَرَحْمَةُ اللهِ وَبَرَكَاتُهُ وَمَغْفِرَتُهُ وَطَيِّبُ صَلَوَاتِهِ