Rashi on Genesis 19:26/רש"י על בראשית י״ט:כ״ו

1ותבט אשתו מאחריו AND HIS WIFE LOOKED BACK FROM BEHIND HIM —behind Lot.

2ותהי נציב מלח AND SHE BECAME A PILLAR OF SALT — By salt had she sinned and by salt was she punished. He (Lot) said to her once: “Give a little salt to these strangers” and she answered him, “Do you mean to introduce this bad custom, also, into our city?” (Genesis Rabbah 50:4).