Yevamot 77 - Was David legitimate?
We mentioned that a female convert from the nation of Moav may marry into the congregation of Israel . This had a direct relevance to King David's provenance.
When Shaul offered his armor to David in order to fight Goliath, he noticed how it miraculously shrank to fit David's size. He then ordered Avner to ask who David's father was. But everyone knew David's father, because he had six hundred thousand people (students) at his command! Rather, he was asking if David was from Peretz, fit to be a king, or from Zerach - just a noble.
Here Doeg intervened and said that they should rather inquire about David's legitimacy altogether, for he came from Ruth, the princess of the people of Moav. Avner defended David: only male converts from Amon have a problem. Doeg countered: then only a male mamzer has a problem, but not female! They kept analyzing the reasons, but Doeg, who was very smart, cornered Avner, who then went to a central yeshiva for support. There, too, Doeg was victorious with his logic.
At this point, Yeter intervened. He girded himself with a sword and said that whoever does not accept the law of female converts from Moav being legitimate would be killed with this sword. However, he also added that he has heard this law from Shmuel the Prophet. He was believed. But why? Wasn't he in the heat of the argument and therefore suspect of lying? - Because Shmuel and his court were alive, and this rule could have been easily verified.
Art: The Cavalier's Sword by William A. Breakspeare