成唯識論演祕 2.922

2.922

種為果相，果相種狹。唯業所招方名為果，即是第八親名言種，是果種體。言唯業果，釋所以也。

The seed serves as the fruit-mark, and the fruit-mark seeds are narrow. Only what is beckoned by karma is called the fruit; that is precisely the eighth's direct nominal seeds, which are the substance of the fruit-seeds. The statement "only the fruit of karma" explains the reason.