I think it was Josephus.
Are you saying Josephus is not credible?
Josephus is definitely credible, but I don’t think the name Cuzar/Kozar/Khazar shows up in his works. The rise of the Khazar kingdom would also have been ahead of his time. However, that name does show up as the son of Togarmah, the grandson of Japheth, in documents dating from the Middle Ages and shortly afterwards.
Josephus never spoke of Kozars or Khazars. There is no evidence of them existing until the 4th century. If you want to find out about Khazars, start with http://www.khazaria.com