Musar for Genesis 25:31

Even when Esau wanted some of Jacob's soup, נזיר, prior to the transaction involving the sale of the birthright, it was Esau who practiced extreme deception as Rashi explains (25,29). Rashi says that the dish consisted of red lentils. How did Rashi know this? Or, at least, who told Rashi this? Rashi was not in the habit of making such a statement without quoting his source!