Yes, it was authentically reported from the Prophet (peace be upon him) that he used to
begin letters with Salam, including his letter to
Heraclius that was related in the Two Sahih (authentic) Books of Hadith (i.e. Al-Bukhari and Muslim) and others, in which he said,
In the Name of Allah, the Most Gracious, the Most Merciful. From
Muhammad, Allah's Slave and His Messenger, to
Heraclius the Emperor of Rome. Peace be upon those who follow guidance. I (Part No. 24; Page No. 133) invite you to embrace Islam...
May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.