Allah tells us that after Ibrahim had calmed down from the excitement of this good news, he started to ask them why they had come to him. They said,
(We have been sent to a guilty people.) meaning the people of Lut. They told him that they were going to save the family of Lut from among those people, except for his wife, because she was one of those who were doomed. Thus it was said,
(Except for his wife, of whom We have decreed that she shall be of those who remain behind.) i.e., she was one of those who would be left behind and will be destroyed.