Al-Adab Al-Mufrad #1004

(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."