元史
During the reign of Möngke, Batu's prestige as kingmaker and viceroy of the great khans in the west reached its height. Even so, Batu allowed Möngke's census takers to operate freely in his realm. According to a Muslim chronicle and the History of Yuan, Batu had free access to the imperial treasury. Of all the grandsons of Genghis Khan, he and Möngke seemed most capable and the most dedicated to following the spirit of Mongolian law.