I mean like, the first serious scientific source confirming them as mammals. I remember reading somewhere that Aristotle knew they couldn't be fish and concluded they were mammals. Perhaps I'm not googling correctly but I can't find any sources to answer my question.

I mean, there have probably been smart people since the Ancient times that found a beached dolphin or whale and realized that it was breathing and had no gills, but when did some actual taxonomer say "wait a minute"?