Who said, "God is not a man, that He should lie"?

Answer: Balaam

Balaam spoke these words in an oracle to Balak son of Zippor, who had hired him to curse Israel.

“Then Balaam lifted up an oracle, saying: “Arise, O Balak, and listen; give ear to me, O son of Zippor. God is not a man, that He should lie, or a son of man, that He should change His mind. Does He speak and not act? Does He promise and not fulfill?”— Numbers 23:18-19 (BSB)