VBM The Burial of Yaakov By Harav Yaakov Medan

I. And the time drew near that Yisrael must die; and he called his son Yosef, and said unto him: “If now I have found favor in your sight, put, I pray you, your hand under my thigh, and deal kindly and truly with me; bury me not, I pray you, in Egypt. But when I sleep with my fathers, you shall carry me out of Egypt, and bury me in their burying-place” And he said: “I will do as you have said.” And he said: “Swear unto me.” And he swore unto him. And Yisrael bowed down upon the bed's head. (Bereishit 47:29-31) Yaakov makes Yosef swear with his hand under his thigh that he will bury him with his fathers. Why is an oath necessary? And why does the oath require that Yosef put his hand under Yaakov's thigh?