Musnad Abu Ya'la al-Mawsili 111:4

Suwayd ibn Sa'id narrated to us, al-Walid ibn Muhammad al-Muqri narrated to us, from al-Zuhri, from 'Ubayd Allah ibn 'Abd Allah ibn 'Utbah ibn Mas'ud: That the Messenger of Allah (may Allah bless him and grant him peace) wrote to Caesar inviting him to Islam. He sent his letter with Dihyah al-Kalbi, and the Messenger of Allah (may Allah bless him and grant him peace) commanded him to deliver it to the governor of Busra so that he would deliver it to Caesar. The governor of Busra delivered it to Caesar. When Allah relieved Caesar of the armies of Persia, Caesar vowed that he would walk from Homs to Aelia (Jerusalem) because of what Allah had tested him with in that regard. When the letter of the Messenger of Allah (may Allah bless him and grant him peace) reached Caesar, he said when he read it: "Seek out whether there is anyone here from his people so that we may ask them about the Messenger of Allah (may Allah bless him and grant him peace)."