Genesis 25:29-33

29One day, while Jacob was cooking some stew, Esau came in from the field and was famished. 30He said to Jacob, “Let me eat some of that red stew, for I am famished.” (That is why he was also called Edom.1)

31“First sell me your birthright,” Jacob replied.

32“Look,” said Esau, “I am about to die, so what good is a birthright to me?”

33“Swear to me first,” Jacob said.

So Esau swore to Jacob and sold him the birthright.