It is narrated from Hazrat Anas (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) wrote separate letters to Khosrow, Caesar, and the king of Dumat al-Jandal, Ukaydir, in which they were invited towards Allah.
Hadith Referenceمسند احمد / مسند المكثرين من الصحابة / 12355