Now there was on that spot in old times a great and noble city called Cambaluc [Beijing], which is as much as to say in our tongue "The city of the Emperor." But the Great Kaan was informed by his Astrologers that this city would prove rebellious, and raise great disorders against his imperial authority. So he caused the present city to be built close beside the old one, with only a river between them. And he caused the people of the old city to be removed to the new town that he had founded; and this is called Taidu.
Source:
Polo, Marco: The Travels of Marco Polo. Book 2, Chapter 11: Concerning the city of Cambaluc.
Further Reading:
I should have phrased that better.
Do we know the reason given by the astrologers? What sign prompted the belief that a rebellion was about to occur?