Genesis 42:8

(8) Joseph knew.—As this is twice repeated, some suppose that Joseph (in Genesis 42:7) had only a suspicion, from their dress and appearance, that these Canaanites were his brethren; but that when they spake the Hebrew tongue (comp. Genesis 42:23), every doubt was removed. They would not recognize him, as he used the Egyptian language, was clad in a white linen dress, and being but seventeen when sold, had during the twenty years of separation changed in appearance much more than they had.