The Ruling on the Prayer of One Whose 'Awrah has become Uncovered
Question :
Sometimes I pray by covering myself with a towel and nothing of the 'Awrah is revealed, but when I prostrate, a little of my knee and what is above is revealed. Bearing in mind that I am alone and there is no one to see me, what is the ruling on this?
Answer:
It is not permissible to reveal any part of the 'Awrah during prayer, whether it be obligatory or non-obligatory. And the Awrah for a man is defined as from the navel to the knees; therefore, it is essential to cover it, and whenever a part of the knee or above it is revealed due to the requirements of the prayer (bowing, prostrating etc.) whether the worshipper is alone or there are people with him, and even if he is in a darkened house or the like he must cover his 'Awrah with something which covers the skin and does not reveal anything of it; and he should not content himself with light, translucent or short clothing which becomes rolled up when bowing or prostrating causing something above the buttocks to be revealed or the thighs, or the knees, regardless of whether the garment is an Izar, short Sarawil,a Jubbah, a Rida' a towel or the like.
Source:
Ibn Jibreen
Fatawa Islamiyah, Vol. 2 Page 200