Yosef Karo

“ Paragraph 1 — The fourteenth [of Nissan] that falls on the Sabbath - we check [for leaven] on the eve of the thirteenth, and we burn it all before the Sabbath, and we leave over food for two meals needed for the Sabbath, since the third meal takes place after mincha, and then one is unable to make a meal of either matzah or leaven, but only with rich matzah. One must do the third meal before the tenth hour. ”