From where does the custom of eating a special meal - Melave Malka - come?

It is customary on Saturday evening to eat a meal in honor of the departure of Shabbat known as Melave Malka - literally, "Escorting the (Shabbat) Queen." As King David was foretold that he would pass away on Shabbat, each Saturday evening he had a meal celebrating another week of life. According to our Sages, the luz bone - from which the body will be formed at the time of the resurrection of the dead - is nourished only from food eaten at the Melave Malka. The Talmud states that eating something hot on Saturday evening is medicinal and helps as an antidote for depression. Another source explains that telling a story about the Baal Shem Tov [some say any righteous person] is beneficial for livelihood.