Hi, it's a story that a young man saved a group of ants, later in an important examination, he forgot to write a point, an ant appeared on the test paper, making up for his mistake. 

I think the story is made up to show that good man will be rewarded. It is not very famous though.

Refer to: https://baike.baidu.com/item/%E8%9A%82%E8%9A%81%E6%8A%A5%E6%81%A9/2213804