Charles Ellicott Commentary 1 Samuel 12:5

"And he said unto them, Jehovah is witness against you, and his anointed is witness this day, that ye have not found aught in my hand. And they said, He is witness." — 1 Samuel 12:5 (ASV)

The Lord is witness. —Then Samuel again, with increased solemnity, called the Eternal in the heavens above and His anointed king then standing by his side to witness what the people had just acknowledged concerning his scrupulously just rule.

And they answered, He is witness. —And the assembly of Israel, again with one voice, shouted, Yes, He is witness.