John Gill Commentary Deuteronomy 9:20

"And Jehovah was very angry with Aaron to destroy him: and I prayed for Aaron also at the same time." — Deuteronomy 9:20 (ASV)

And the Lord was very angry with Aaron, to have destroyed him , &c.] For complying with the request of the people in making a calf for them, and for that miserable shift he made to excuse himself; which so provoked the Lord, that he threatened to destroy him, and he was in danger of being cut off, had it not been for the intercession of Moses:

and I prayed for Aaron also the same time : who either was included in the general prayer for the people, (Exodus 32:31) or a particular prayer was made for him, though not recorded, and which also succeeded.