Shulchan Arukh, Orach Chayim 157/שולחן ערוך·אורח חיים קנ״ז

1Laws of When To Establish One's Meal. Containing 1 Se'if: When the fourth hour [of the day] arrives, one should establish his meal. And if he is a Torah scholar and he is occupied with his learning, he should wait until the sixth hour, but he should not delay more than that for it is considered like throwing a stone into a [wine] pouch if he did not eat anything that morning.