Which document stated that if any European nation tried to occupy land in the Western Hemisphere the US would consider it a threat to the nation's peace and safety?

The document you're referring to is the Monroe Doctrine, articulated by President James Monroe in 1823. This doctrine declared that any European intervention in the affairs of the Americas would be viewed as a hostile act against the United States. It established a key principle of U.S. foreign policy, asserting the Western Hemisphere's independence from European colonialism.