Kejadian 42:8

'ויכר יוסף וגו AND JOSEPH RECOGNISED HIS BRETHREN etc. — because, when he left them they were full-bearded (Yevamot 88a). והם לא הכירהו BUT THEY RECOGNISED HIM NOT — because when he left them he had no beard whereas now he had grown a beard. A Midrashic explanation is: ויכר יוסף את אחיו JOSEPH RECOGNISED HIS BRETHREN — Now that they were in his power he recognised them as his brothers and had pity on them, והם לא הכירהו but when he fell into their power, “they did not recognize him” as their brother, by acting towards him in brotherly manner (Genesis Rabbah 91:7).

וַיַּכֵּ֥ר יוֹסֵ֖ף אֶת־אֶחָ֑יו וְהֵ֖ם לֹ֥א הִכִּרֻֽהוּ׃

And Yosef knew his brethren, but they knew him not.