וַיֹּ֣אמֶר יַעֲקֹ֗ב הִשָּׁ֤בְעָה לִּי֙ כַּיּ֔וֹם וַיִּשָּׁבַ֖ע ל֑וֹ וַיִּמְכֹּ֥ר אֶת־בְּכֹרָת֖וֹ לְיַעֲקֹֽב׃
And Jacob said: ‘Swear to me first’; and he swore unto him; and he sold his birthright unto Jacob.
Read halakhic rulings (Jewish law) on Genesis 25:33 from the Mishneh Torah and classical halakhic sources, free online.