Torah Temimah cites Yerushalmi Berachos 9:1.
The heretics asked Rabbi Simlai: How many divinities created the world? He said: Go and ask the first man, as it is stated (Devarim 4:32), “For ask now of the days that are past... from the day that God created man upon the earth” — not “created” (plural) but “created” (singular). They said: But it is written “Bereishis bara Elohim”! He replied: Is “baru” (plural) written? “Bara” (singular) is written.