Scholars differ as to wheter the knee is part of a man’s `awrah (the part of a man’s body that must be concealed in public.
Ibn Qudamah in al-Mughnî says: “The knees themselves do not have to be covered (they are not part of a man’s`awrah). This is the opinion of Ahmad, Mâlik and al-Shâfi`î”
Abû Hanîfah, on the other hand, was of the opinion that the knees are part of a man’s `awrah.
This disagreement stems from linguistic considerations concerning the interpretation of the preposition “to” in the hadîth that says a man must be covered “from the navel to the knee”. The question is whether the phrase “…to the knee” means “up to and including the knee” or merely “up to the start of the knee”.
The stronger opinion is that held by the majority of scholars, which is that the knee is not part of a man’s `awrah.
However, this means that a Muslim man must wear clothing that is of sufficient length to cover all of the thigh, whether he is standing, sitting, bowing, in jalsah, or in prostration. His entire thigh must remain covered at all times.
Therefore, shorts that stop just above a man’s knee when he is in a standing position are not sufficient, because his lower thighs will be exposed whenever he sits down and whenever he bows and prostrates in prayer.
And Allah knows best.