avot patriarch fathers - Why did Yaakov ask Yosef to swear in the way he did?

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In Bereshis 47:29 Yaakov asks Yosef to "put his hand under his thigh" (a euphemism) and swear not to bury him in Egypt. Here, unlike by Avraham, there is no indication that the mitzvah of milah is more important to Yaakov than any other (such as learning torah or gid hanasheh). Why then did he ask Yosef to swear in that way?