The Wikipedia page on Thomas Becket contains this excerpt:
Upon hearing reports of Becket's actions, Henry is said to have uttered words that were interpreted by his men as wishing Becket killed. The king's exact words are in doubt and several versions have been reported. The most commonly quoted, as handed down by oral tradition, is "Will no one rid me of this turbulent priest?", but according to historian Simon Schama this is incorrect: he accepts the account of the contemporary biographer Edward Grim, writing in Latin, who gives us "What miserable drones and traitors have I nourished and brought up in my household, who let their lord be treated with such shameful contempt by a low-born cleric?" Many variations have found their way into popular culture.
So Simon Schama appears to be using the account from Edward Grim, who was an eye-witness of the assassination; but not -- this should be pointed out -- of Henry's utterance. So even Schama's primary source is already repeating hearsays. Compounding the issue is that it is improbable that Henry was speaking in Latin (the language used by Grim); Henry knew Latin, but for an exasperated curse he would have more naturally used French (at least the French flavour spoken in Norman court at that time). Moreover, "embellishing" discourse was also a normal practice for historians at that time (nobody in the 12th century believed that reporting the exact words, as they were said, was important).