פירוש על קידושין 138:5

Rav Yehudah said that the lineage of most countries is dough in comparison with Israel, meaning it is all muddled. But Israel’s lineage is more muddled than Babylonia. This works well with Abaye who said that they went up to Israel voluntarily and that Ezra did not sort their lineages out before they went up. There was no oversight as to who could marry whom. But to Rava who said that Ezra forced them to go, Ezra sorted their lineage out. So why is Israel like dough in comparison to Babylonia?
The answer is that while that generation knew its lineage, subsequent ones did not.