It is commonly thought that 3 INT is the minimum amount required in order to take class levels, but where does it state such?
It is often inferred or implied (animals have intelligence 1 or 2, and if they manage to obtain a score if 3 or more they become magical beasts, which have been noted to be able to take class levels and no examples of animals with class levels exist that I'm aware of).
It has been established that non- intelligent creatures cannot take class levels (normally, but there's a trick for that).
So, where does this commonly accepted idea come from? Looking primarily for any 3rd edition quote and/ or reference. If it only exists in another edition, then that's fine too.