In the name of Allah the Beneficent the Merciful

Peace, blessings and mercy of Allah be upon you.

Many Muslim praying brothers wear pants, and many of them does not use belts to keep them tight from falling down. Unfortunately, we find many of them, when they kneel or prostrate, their private parts show, as it is considered that the private part lies in the area between the navel and the knee. Then, what is the ruling about the prayers of such people? And what is your advice to the Imams of the mosques in instructing people like those, especially when you advise some individual person, it would make the matter very embarrassing to him? Jazakumu Allahu Khairan.

In the name of Allah the Beneficent the Merciful

Peace, blessings and mercy of Allah be upon you.

Praise be to Allah, prayers and peace of Allah be upon His kind Messenger, family members, companions and true followers to the Day of Judgment.

Covering the private parts is a condition for the prayer to be correct and valid, and what you have mentioned in your question, is a proof that ignorance still prevails among some Muslims, and they are not to be excused for that by Allah the Almighty. The Imam is supposed to instruct worshippers during the Friday sermons about this very important issue. The Maliki scholars of Fiqh school rule that the prayer is absolutely devoid if the private parts of a worshipper show while praying, but the Shafi`i and Hanbali doctors maintain that the prayer is still valid and correct if the private parts are uncovered suddenly, for example, by the act of the wind and it is done without intention, and they should be restored to be covered immediately. However, the Hanafi doctors rule that the prayer would be incorrect if about one quarter of the private parts is uncovered for a time sufficient to do one kneeling (rak`ah), for example. Allah knows best.