In short, his mother, Sarah, was an orphan living in Lod. Her neighbor was a non-Jew who was impressed by her middot. One day, a Roman legion passed through Lod and kidnapped Sarah, planning on making her a slave in Rome. Her neighbor decided to try to save her. He followed her all the way to Rome. Shortly after arriving in Rome, he wandered outside of the city. He came upon a hunter whom he recognized as the son of the emperor. The hunter attempted to kill a lion, but the lion attacked him. The man grabbed a stick and beat the lion, until it turned and ran away. The prince was badly injured, and so the man did for him what he could and then carried him back to the city. The royal doctors managed to save his life. Grateful, the emperor asked the man what he could do for him. The man requested he let Sarah return with him to Judea. The emperor agreed. Upon returning, the man, still impressed by Sarah's middot and faith, decided to convert. He changed his name to Yosef and studied for five years. After that, he married Sarah. Shortly after she became pregnant, Yosef passed away. Sarah decided to move to Yerushalayim, where she became a cook in Kalba Savua's household. While there, her son Akiva was born, and he grew up and became a shepherd in the household (and the rest is history...).
I had never heard this story before. Does anyone know where this appears?