A man named Mara Bar-Serapion wrote to his son anywhere after 73 AD and before 165 AD. There is even a little chance that 73 AD could be pushed back to be after 135 AD.
Mara Bar-Serapion wrote to his son in prison pointing out that those who punish wise men receive disaster and fail at stopping their teaching:
What advantage did the Athenians gain from putting Socrates to death? Famine and plague came upon them as a judgment for their crime. What advantage did the men of Samos gain from burning Pythagoras? In a moment their land was covered with sand. What advantage did the Jews gain from executing their wise king? It was just after that that their kingdom was abolished. God justly avenged these three wise men: the Athenians died of hunger; the Samians were overwhelmed by the sea; the Jews, ruined and driven from their land, live in complete dispersion. But Socrates did not die for good; he lived on in the teaching of Plato. Pythagoras did not die for good; he lived on in the statue of Hera. Nor did the wise king die for good; he lived on in the teaching which he had given
The obvious problem with this passage is to work out who this "wise king" is. If Jesus is the wise king, then we have a pretty good external source for some facts about his life that can be meshed up with the gospel accounts. But is Jesus this "wise king"?