John Gill Commentary 1 Kings 18:17

"And it came to pass, when Ahab saw Elijah, that Ahab said unto him, Is it thou, thou troubler of Israel?" — 1 Kings 18:17 (ASV)

And it came to pass when Ahab saw Elijah
As soon as he came up to him, and knew who he was; Abarbinel thinks, because his hair was grown so long that Ahab did not know him certainly, and therefore put the following question:

that Ahab said unto him, art thou he that troubleth Israel ?
by opposing the religion of Baal, which prevailed among them; but chiefly rain being withheld from them according to his word, and at his prayer.