What you have heard is a bit overstated. The best example of such a testimony is Josephus. The problem is that in the Greek text we have today, Josephus calls Jesus “the Christ,” and nearly everyone (including conservative Christian scholars such as F. F. Bruce) agrees that Christians have tampered with the text (early Christians writing before the fourth century did not cite this testimony). There is a tenth-century Arabic text citing a somewhat different version of the passage which sounds more like what a non-Christian might say–however, the idea that this is the authentic, original version of the passage has been challenged. The
wikipedia article on this seems pretty fair to me. There’s another passage, dealing with John the Baptist, in which Josephus mentions Jesus briefly. So I think Josephus is evidence that Jesus really lived (something very few scholars doubt, by the way), but there are difficulties involved in citing the passage as we have it.
There really aren’t any other testimonies quite like this. Tacitus and Suetonius, Roman historians who wrote in the late first and early second centuries, refer to Christians and to their origin as followers of Jesus (as does Pliny the Younger in the early 2nd century, asking Trajan for advice on how to deal with Christians). However, this doesn’t necessarily reflect independent knowledge on their part–it appears to be what was commonly known about Christians, probably based on what Christians themselves claimed.
Some medieval Byzantine sources include fragments from early Christians citing ancient historians who allegedly wrote about the eclipse of the sun at the time of Jesus’ death. However, given the form in which we have these testimonies, it’s not clear whether the pagan writers themselves associated this event with Jesus or whether they simply wrote about an eclipse which the
Christian writers identified with the death of Jesus.
I may have missed something, but these are the principal early non-Christian references to Jesus I can think of.
Edwin