Genesis 42:7

And Joseph saw his brethren, and he knew them, but made himself strange unto them, and spake roughly unto them, he literally spoke to them hard things; and he said unto them, Whence come ye? And they said, From the land of Canaan to buy food. It was an easy matter for Joseph, even after the lapse of some twenty years, to recognize his brothers; their number, their language, their clothing, their manner indicated at once who they were. But not one of them would have looked for Joseph in the person of this despotic Egyptian, whose dress and language were entirely foreign to them. Joseph purposely spoke harshly to them, in order to sound them out, to find whether their hearts had changed in the last two decades. Though he still loved them, his treatment would provide some wholesome discipline for them.