External reference of Jesus's resurrection? After that he appeared to more than 500 brothers at one time, most of
  whom are still with us, though some have fallen asleep in death. - 1
  Cor 15:6


I'm trying to find some external/non-biblical references about Jesus' resurrection. The quoted verse says there were a lot of witnesses to tell Jesus has been resurrected, so I'm wondering if any of them wrote something that referenced this event ?
According to The New Evidence That Demands a Verdict by Josh McDowell, p125, Josephus, who died after 100AD, wrote of Jesus in his Jewish Antiquities, "...for he appeared to them alive again the third day; as the divine prophets had foretold these and then thousand other wonderful things concerning him." (Antiquities, XVIII, 33).

However, those words are probably a later addition. 

The same book notes several creedal formations mentioning Christ's resurrection that made their way into the epistles after first being passed around orally (same book, p127). 

Ignatius, whom Christian tradition identifies as a disciple of Peter, Paul, and John, wrote about the resurrection of Jesus (same book, p131). The book goes on to cite several more early writers who wrote of Jesus' resurrection, like Quadratus, Aristides, Justin Martyr, Hegesipus.