פירוש על קידושין 137:9

The rabbis read the word “of his” as referring to a case where an Israelite marries a mamzeret. We might have thought that in such a case the lineage follows the father’s house and the child is not a mamzer. They read the word “of his” as excluding such a person from entering the community of Israelites. He does not follow his father’s lineage.