Matthew 12:40
For as Jonas was three days and three nights in the whale's belly; so shall the Son of man be three days and three nights in the heart of the earth.
since he knew there was a center to the earth, then it shows he knew the earth was round
The New Testament is dated to the first century CE. A long time earlier Eratosthenes 276 BC – c. 195 BC calculated the circumference of the Earth surprisingly accurately.
Even before that the Old Testament records:
Isaiah 40:22 22 He sits enthroned above the circle of the earth, and its people are like grasshoppers. He stretches out the heavens like a canopy, and spreads them out like a tent to live in.
Although to be fair this could be relating to the Mesopotamian concept of the earth as a flat disc surrounded by a spherical sky. The earliest mention of a spherical shaped earth seems to be again from the Greeks around the sixth century BC. One source says Pythagoras was the first to suggest the earth was spherical. The point being though that Jesus was most certainly not saying anything that wasn't already well known around the ancient world.