首楞嚴義疏注經 4.157

4.157

眾生覺隱如金雜鑛，諸佛覺顯如金一純。已入果海，不重為因，故云更不成雜。然因果雖殊，覺性平等。

The enlightenment of sentient beings being concealed is like gold mixed with ore; the enlightenment of buddhas being manifest is like gold made purely one. Having already entered the ocean of the fruit, it does not become a cause again; hence it says "never becomes mixed again." However, although cause and fruit are distinct, the nature of enlightenment is equal.