Musannaf Ibn Abi Shayba 28:1018

'Abd al-Rahim ibn Sulayman narrated to us, from Zakariyya, from Khalid ibn Salamah, from 'Amir, who said: Khalid ibn al-Walid wrote from al-Hirah to the marzbans (governors/commanders) of Persia: In the name of Allah, the Most Gracious, the Most Merciful. From Khalid ibn al-Walid to the marzbans of Persia: Peace be upon those who follow the guidance.