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In parshas Vayechi 47 29 Yaakov Avinu tells Yosef to place his hand by his thigh. We find this expression used by Avraham Avinu in Chayei Sarah 24 2 with an explenation by Rashi that an object used for a mitzvah needed to be held for the shvua and Avraham chose his mitzvas mila because it was his first mitzva, and he got it through pain, and it was precious to him. None of this applies to Yaakov, so why did he choose it? There is even a Medrash that says Yaakov was born without a foreskin so there is no object of mitzva at all.