Rashi on Genesis 12:14/רש"י על בראשית י״ב:י״ד

1ויהי כבא אברם מצרימה AND IT CAME TO PASS WHEN ABRAM WAS COME INTO EGYPT —It should have said, “when they were come into Egypt”; but the use of the singular teaches us that he hid her in a chest, and when they demanded the custom dues they opened it and discovered her (Genesis Rabbah 40:5).