Narrated `Abdullah bin `Abbas: Allah's Apostle wrote to Caesar and invited him to Islam and sent him his letter with Dihya Al-Kalbi whom Allah's Apostle ordered to hand it over to the Governor of Busra who would forward it to Caesar. Caesar as a sign of gratitude to Allah, had walked from Hims to Ilya (i.e. Jerusalem) when Allah had granted Him victory over the Persian forces. So, when the letter of Allah's Apostle reached Caesar, he said after reading it, 'Seek for me any one of his people! (Arabs of Quraish tribe) if present here, in order to ask him about Allah's Apostle.

Narrated `Abdullah bin `Abbas: Allah's Apostle wrote a letter to Caesar saying, "If you reject Islam, you will be responsible for the sins of the peasants (i.e. your people).