I was reading up on this, and I have seen several sources that say he did comment, but I can't find anything other than footnotes going to other footnotes which seemingly go nowhere (one of them went to World Religions: Ancient Traditions, which reference a James Legge translation of Mencius, however i was unable to find the passage within my copy of the Mencius).
All of this is very irksome for various reasons, so I'm going to cross post this to a couple of subreddits as well in hopes of finding an answer.
You are looking for two historical records in particular.
One is from Meng Zi (Mencius, the one you are looking for), in the chapter about King of Wei. Meng Zi claimed that Confucius praises the craftsman that first invented using puppets in the substitution of actual humans, replacing humans as burial sacrifice. The chapter records an exchange between Meng Zi and King of Wei, during the conversation Meng Zi invoked some lines that was supposedly said by Confucius, whether the lines are actually from Confucius or just Meng Zi using his name to add weight to his own arguments, is up to debate. Not that the line Meng Zi quoted in that chapter is not present in the Analects.
The second is from Book of Rites. Confucius explains why he prefers using replicas and puppets instead of actual humans and actual items of value. Confucius said that, "涂车、刍灵，自古有之，明器之道也" (replica carts and wooden puppets are of ancient origin, an established way of burial offerings).
In general, the argument to human sacrifice is both moral and pragmatic. Moral, killing people is not good. Pragmatic, waste of human resources, especially during the warring period of that era.