Is it true what is circulated about the 'awrah of a female slave being between the navel and the knee, and that 'Umar used to strike those among them who wore the hijab? What is the evidence for those who say this, and what is the wisdom behind the imposition of hijab on free women but not on female slaves?

There are no male or female slaves nowadays, and a Muslim should not occupy their time with their rulings. Islam has responded to the proponents of absolute freedom, emphasizing that its legislations are perfect and unparalleled.

Regarding the awrah (intimate parts) of a female slave, scholars have differed. The Maliki and Shafi'i schools held that it is from the navel to the knee. The Hanafis considered it to be like the awrah of a free woman in front of her mahrams (unmarriageable kin). The Hanbalis viewed it as similar to the awrah of a free woman, with the permissibility of looking at what is permissible to look at in a free woman.

Shaykh al-Islam Ibn Taymiyyah clarified that the hijab (veil) is specific to free women and not female slaves. He explained that a female slave would expose herself because she was often engaged in service, and the Shariah permitted her to uncover her head, arms, and legs to alleviate hardship. Some jurists believe that if a female slave's unveiling leads to fitnah (temptation), then she must wear the hijab, just as lowering the gaze from her is obligatory.

As for a married female slave, nothing of her should be visible to her master except what is normally visible of her during work.

Regarding a buyer, he may look at the female slave's face, head, and what is visible of her over her clothes during work; he should not examine most of her body.

These rulings illustrate that Islam came with what achieves benefits and repels harms, and that its rulings are the wisest possible.