Dear Mr. Kahan,

Hello there and thanks for your question. There is a Midrash that Yishai (Dovid's father) was always worried that one day his family would be disqualified from marrying into Kehal Yisroel, because of his Moabite lineage. He therefore decided to marry his maid and have at least one child who would be permitted to marry into Kehal Yisroel after he would be freed. The maid, didn't want to go along with this idea, and told Yishai's wife to take her place instead. The result was that Yishai's wife gave birth to Dovid haMelech. Yishai however, who was unaware of the switch, was sure that this child was an illegitimate one. He thus sent him out to tend the sheep, hoping that he would be killed by wild animals. Many years later, Dovid related that he killed the lion and the bear. This showed that Dovid was protected from harm, thus proving that he was a legitimate child.

Perhaps this is the story you were looking for.

All the best.

Y. Landy