There are variations of this story in Zen books.
Once there was a monk who lived in a hut in the compound of a very wealthy and pious patrons. He earned his meals by helping take care of the yard. The couple had a daughter. One day the daughter announced that the monk violated her and she was pregnant with his child.
Naturally, the couple were furious. When confronted all he said was "Oh.."
As the girl was still a child herself, they waited until the baby was born, gave the baby, a boy, to the monk and shuttled him as far away from their province as possible.
The monk supported the child by doing manual labor wherever it was possible to get work.
When the boy was seven years old, a servant asked him and the boy to have dinner with his employees. The monk obliged, and together with the boy showed up at the house indicated by the servant.
As it turned out, his hosts were his former benefactors, traveled all the way from their province. The monk did not say anything. The boy was delighted to be in such a feast.
After dinner, the hosts told the servants to bathe and clothe the boy, give him all the toys and play with him for a little while, and the monk was left alone with the hosts.
The hosts explained that a few days ago, their daughter admitted that the father of the child is the son of the a clan whom they particularly disliked and she did not tell them about him because she was afraid of what they would do, but now that there was no longer as much animosity between the clans, she could tell them, and they have arranged for her marriage to the father of the child. They wanted their grandson back. The monk said "Oh.."
When the boy came back, to see his "father", the monk took him to the yard and told him everything that the patrons told him and told him they have come to take him with them. The boy cried, but he understood.
The monk thanked his hosts and left.