A lot of older sources will probably mention a great 'Yasa,' of Genghis Khan which was thought to be a sort of law codex created by Genghis Khan himself. However later scholars have doubted its form if not its existence. It might still contain sayings from Genghis Khan but on a whole it was more likely to be a codex of mongol custom which was naturally developing for years.

Source: David Morgan, the “great YASA of chinggis khan”revisited