What is the evidence that a woman's awrah before her mahrams is from the navel to the knee?

Some Shafi'is hold the view that a man may look at his unmarriageable kin (mahārim) at what is above the navel and below the knee, provided there is no fear of temptation (fitna) and his desire is not aroused. They cite as evidence the verse: ﴿وَلَا يُبْدِينَ زِينَتَهُنَّ إِلَّا لِبُعُولَتِهِنَّ﴾ (and not expose their adornment except to their husbands), arguing that the adornment mentioned refers to everything except what is between the navel and the knee. They also argue that since unmarriageable kin are unlawful for marriage, it is permissible to look at them, just as one man may look at another.

However, the preponderant view is that the verse permits the exposure of what commonly appears and is difficult to conceal, in order to remove hardship from the woman in her home. It does not mean exposing the abdomen, back, and other such parts. Furthermore, drawing an analogy with a man looking at another man is an invalid analogy due to the difference in circumstances, as there is a risk of temptation when a man looks at his unmarriageable kin.

The correct view is that a man may look at what commonly appears of his unmarriageable kin, such as the neck, head, hands, and feet. However, he is not permitted to look at what is usually concealed, such as the chest, back, and similar parts.