C. F. Keil and Franz Delitzsch on 1 Samuel 12:5

1Sa 12:5

To confirm this declaration on the part of the people, he then called Jehovah and His anointed as witnesses against the people, and they accepted these witnesses. כּל־ישׂראל is the subject to ויּאמר; and the Keri ויּאמרוּ, though more simple, is by no means necessary. Samuel said, “Jehovah be witness against you,” because with the declaration which the people had made concerning Samuel’s judicial labours they had condemned themselves, inasmuch as they had thereby acknowledged on oath that there was no ground for their dissatisfaction with Samuel’s administration, and consequently no well-founded reason for their request for a king.