The extent of covering the Satr in prayer
A man's Satr is from the navel to below the knee, which is obligatory (Wajib) to cover inside and outside of prayer. The extent of a man's Satr that has been described consists of eight parts (limbs) according to the jurists (Fuqaha). If one-fourth of any one of these parts remains uncovered for the duration of one Rukn, i.e., equivalent to reciting three Tasbeehat, the prayer will become invalid (Fasid). (Aham Masail / Vol: 1 / Pg: 34)