Deuteronomy 23:9

בנים אשר יולדו להם דור שלישי וגומר THE CHILDREN THAT ARE BORN UNTO THEM [MAY COME INTO THE ASSEMBLY] IN THEIR THIRD GENERATION — other nations, however, since they did not sin against you may be admitted at once if they acknowledge the tenets of Judaim. — Thus you learn that he who causes a man to sin does him greater harm than if he kills him, for he who kills him, kills him only as regards this world, while he who causes him to sin puts him out of this world and the world to come. Therefore Edom, though he met them with the sword was not to be abhorred utterly, and similarly the Egyptians who drowned them (their male children), while those (the Ammonites and Moabites) who caused them to sin were to be utterly abhorred (Sifrei Devarim 252:3).

בָּנִ֛ים אֲשֶׁר־יִוָּלְד֥וּ לָהֶ֖ם דּ֣וֹר שְׁלִישִׁ֑י יָבֹ֥א לָהֶ֖ם בִּקְהַ֥ל יְהֹוָֽה׃ {ס}        

The children that are begotten of them shall enter into the congregation of the Lord in their third generation.