John Gill Commentary Deuteronomy 9:12

"And Jehovah said unto me, Arise, get thee down quickly from hence; for thy people that thou hast brought forth out of Egypt have corrupted themselves; they are quickly turned aside out of the way which I commanded them; they have made them a molten image." — Deuteronomy 9:12 (ASV)

And the Lord said to me
The omniscient God, who knew what was doing in the camp of Israel, though Moses did not, of which he informs him:

arise, get you down quickly from hence ;
from the mount where he was; and the word "arise" does not suppose him to be sitting or lying along, neither of which postures would have been suitable, considering in whose presence he was; but is only expressive of urgency and haste of his departure; it is not used in (Exodus 32:7)

for your people, which you have brought forth out of Egypt, have
corrupted themselves ;
their way, as the Targum of Jonathan; that is, by idolatry, than which nothing is more corrupting and defiling; the Lord calls them not his people, but the people of Moses, being highly displeased with them; and ascribes their coming out of Egypt to Moses the instrument, and not to himself, as if he repented of bringing them from thence:

they are quickly turned aside out of the way which I commanded them :
it being but about six weeks ago, that the command forbidding idolatry, the sin they had fallen into, had been given them:

and they have made them a molten image ;
the image of a calf made of melted gold.