פירוש על ביצה 30:6

Rav Ashi says – They ordain it not on account of the honor of Shabbat, but rather the honor of Yom Tov.

So people will say "do not bake on Yom Tov for Shabbat" – unless the person begins to cook when it is still day, because then it is finishing what you started, but you cannot start.