Sahih Bukhari Volume 4, Book 56, Hadith 2941

حَدَّثَنَا إِبْرَاهِيمُ بْنُ حَمْزَةَ، حَدَّثَنَا إِبْرَاهِيمُ بْنُ سَعْدٍ، عَنْ صَالِحِ بْنِ كَيْسَانَ، عَنِ ابْنِ شِهَابٍ، عَنْ عُبَيْدِ اللَّهِ بْنِ عَبْدِ اللَّهِ بْنِ عُتْبَةَ، عَنْ عَبْدِ اللَّهِ بْنِ عَبَّاسٍ ـ رضى الله عنهما ـ أَنَّهُ أَخْبَرَهُ أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم كَتَبَ إِلَى قَيْصَرَ يَدْعُوهُ إِلَى الإِسْلاَمِ، وَبَعَثَ بِكِتَابِهِ إِلَيْهِ مَعَ دِحْيَةَ الْكَلْبِيِّ، وَأَمَرَهُ رَسُولُ اللَّهِ صلى الله عليه وسلم أَنْ يَدْفَعَهُ إِلَى عَظِيمِ بُصْرَى لِيَدْفَعَهُ إِلَى قَيْصَرَ، وَكَانَ قَيْصَرُ لَمَّا كَشَفَ اللَّهُ عَنْهُ جُنُودَ فَارِسَ مَشَى مِنْ حِمْصَ إِلَى إِيلِيَاءَ، شُكْرًا لِمَا أَبْلاَهُ اللَّهُ، فَلَمَّا جَاءَ قَيْصَرَ كِتَابُ رَسُولِ اللَّهِ صلى الله عليه وسلم قَالَ حِينَ قَرَأَهُ الْتَمِسُوا لِي هَا هُنَا أَحَدًا مِنْ قَوْمِهِ لأَسْأَلَهُمْ عَنْ رَسُولِ اللَّهِ صلى الله عليه وسلم‏

Narrated By 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 t, 'Seek for me any one of his people! (Arabs of Quraish tribe) if present here, in order to ask him about Allah's Apostle.