Babylonian Talmud, Yevamot 78b-79a

It is written: "And there was a famine in the days of David three years, year after year (Samuel II 21:1)...And David sought the countenance of the God...And God said: ‘It is for Saul and his bloody house, because he put to death the Gibeonites.’" ‘For Saul’, because he was not mourned for in a proper manner; ‘and his bloody house,’ because he put to death the Gibeonites. Where, however, do we find that Saul ‘put to death the Gibeonites’?! The truth is that, as he killed the inhabitants of Nob, the city of the priests who were supplying them with water and food, Scripture regards it as if he himself had killed them [See Samuel I 21:2-7; Samuel I 22:6-18; Joshua 9:23 and 27]…Immediately the king called the Gibeonites, and said unto them...‘What shall I do for you and with what should I make atonement, that you may bless the inheritance of God’? And the Gibeonites said to him: ‘It is no matter of silver or gold between us and Saul, or his house,’ neither is it for us [to put] any man etc...Let seven men of his sons be delivered unto us and we will hang them up unto God etc.’…But, surely, it is written, The fathers shall not be put to death for the children! — R. Hiyya b. Abba replied in the name of R. Johanan: It is better that a letter be rooted out of the Torah than that the Divine name shall be publicly profaned...For passers-by were inquiring, ‘What kind of men are these?’ — ‘These are royal princes’ — ‘And what have they done?’ — ‘They laid their hands upon unattached strangers’ — Then they exclaimed: ‘There is no nation in existence which one ought to join as much as this one. If [the punishment of] royal princes was so great how much more that of common people; and if such [was the justice done for] unattached proselytes, how much more so for Israelites. A hundred and fifty thousand men immediately joined Israel. [Soncino translation]