Musnad Abu Ya'la al-Mawsili 115:230

Ubayd Allah narrated to us, Abd al-Rahman ibn Mahdi narrated to us, Imran al-Qattan narrated to us, from Qatadah, from Anas ibn Malik: That the Messenger of Allah (may Allah bless him and grant him peace) wrote to Chosroes, Caesar, and Ukaydir of Dumah, inviting them to Allah.