Eusebius (265-340 AD), the Christian bishop of Caesarea in Palestine and famous father of church history, wrote in his "History of the Church" that he had seen and translated into Greek from Syriac some significant documents in the Record Office at Edessa. Among them was a copy of a letter which Abgar Uchama wrote to Jesus, and Jesus' reply. Abgar had in his kingdom (in today's northern Syria/southern Turkey) heard of the remarkable cures which Jesus was said to have performed in Jerusalem and asked Jesus to come to him and cure a disorder from which he suffered. Jesus responded that He could not come as He "must complete all that I was sent to do here, and on completing it must at once be taken up to the One who sent me." But he promised to send one of his disciples after He was taken up. Eusebius also tells us that a third document was added to the letters which tells how the disciple named Thaddeus (in Mark 3, 18, but called Lebbeus in Matthew 10,3 or Judas, son of Jakob, in Luke 6, 16 and Acts 1,13) came to Abgar, cured him of his disorder, and was permitted to preach to the citizens of the city. See here how another member of our family first was converted by Thaddeus, but later killed him. 
One would perhaps feel tempted to doubt these claims - had not Eusebius with his scientific work and incredible intellect become such a giant in church history. He was a personal friend of Pamphilos, founder of the famous library in Caesarea, and of roman emperor Constantine the Great, whom he helped convince to become the first Christian emperor and wrote the biography "Vita Constantini" on. He is not to be confused with the Eusebius (of Nicomedia) who baptized Constantine.