In The Description of the World, Marco Polo describes how the Great Khan gave them a "passport" – a tablet made of gold – so that they "should be supplied with everything needful in all the countries through which they should pass – with horses, with escorts, and, in short, with whatever they should require" (public domain).
To make sure the Polos would be given every assistance on their travels, Kublai Khan presented them with a golden tablet (or paiza in Chinese, gerege in Mongolian) inscribed with the words: "By the strength of the eternal Heaven, holy be the Khan's name. Let him that pays him not reverence be killed." The golden tablet was a special passport, authorizing the travellers to receive throughout the Great Khan's dominions such horses, lodging, food and guides as they required.
Learn more: Marco Polo and His Travels