法苑珠林 9.81

9.81

汝欲知之。吾往世時行於無量不可計會眾德之本故。如來宿世奉行如斯。乃能致此三十二大人之相也。

You ought to know this: because when I was in former lives I practiced the roots of countless and incalculable multitudes of virtues, the Tathāgata practiced in this way in past lives and was thus able to obtain these thirty-two marks of a Great Person.