Exodus 34:7

נֹצֵר חֶסֶד לָאֲלָפִים נֹשֵׂא עָו‍ֹן וָפֶשַׁע וְחַטָּאָה וְנַקֵּה לֹא יְנַקֶּה פֹּקֵד עֲו‍ֹן אָבוֹת עַל בָּנִים וְעַל בְּנֵי בָנִים עַל שִׁלֵּשִׁים וְעַל רִבֵּעִים

(G-d) extends mercy to the thousandth, forgiving iniquity and transgression and sin. He cleanses; He will not not cleanse, visiting the sin of the fathers on the children and on the children's children to the third and the fourth.

As familiar as this verse is, parts of it are obscure and even appear contradictory. In his Aramaic translation, Onkelos clarifies it with insertions to fill in the gaps. He renders this verse: “(G-d) extends mercy to the thousandth generation, forgiving iniquity and transgression and sin. He cleanses those who return to His Torah; He will not not cleanse those who do not return, visiting the sin of the fathers on the children and on the children’s children who rebel to the third generation and the fourth generation.”