Isaiah 21:6
"For thus hath the LORD said unto me, Go, set a watchman, let him declare what he seeth."
Cross References (ranked by relevance)
Ezekiel 3:17
Son of man, I have made thee a watchman unto the house of Israel: therefore hear the word at my mouth, and give them warning from me.
Isaiah 62:6
I have set watchmen upon thy walls, O Jerusalem, which shall never hold their peace day nor night: ye that make mention of the LORD, keep not silence,