What is the evidence for restricting the salam to mahrams and elderly women?

Question:
May Allah reward you. Honorable Shaykh, this questioner says: Ibn al-Qayyim (may Allah have mercy on him) mentioned that the stronger position in the issue of greeting women is that one greets elderly women and those who are mahrams, but not others, based on the hadith of Asma bint Yazid and the hadith of the Companions greeting an old woman they passed by. The question is: Where did Ibn al-Qayyim derive the evidence for restricting the salam to mahrams and not others?

Answer:
Fitnah, the fear of fitnah. There is no fitnah with mahrams, and there is no fitnah with an old woman. As for others, there is fitnah in their voice, and some laxity may occur between a man and a woman, which could become the cause of a relationship between them? We seek help from Allah. Nowadays, there is no modesty left, except what Allah wills. It has become the man who is modest, while the woman is not. Yes.