Which prophet warned Ahab before Ramoth-gilead that Israel would be scattered?

Answer: Micaiah

Micaiah son of Imlah saw Israel scattered like sheep without a shepherd and said the Lord had pronounced disaster on Ahab.

“So Micaiah declared: “I saw all Israel scattered on the hills like sheep without a shepherd. And the LORD said, ‘These people have no master; let each one return home in peace.’” So you see, the LORD has put a lying spirit in the mouths of all these prophets of yours, and the LORD has pronounced disaster against you.””— 1 Kings 22:17, 22:23 (BSB)