To expose ‘Satr’ without a genuine reason even when alone is not permissible; and in front of others or in Ṣalāĥ, covering the ‘Satr’ is Farḍ by consensus (Ijmā’). (Baĥār-e-Sharī’at, part 3, vol. 1, pp. 479)
صَلُّوۡا عَلَى الۡحَبِيۡب صَلَّى اللّٰهُ تَعَالٰى عَلٰى مُحَمَّد
Rulings on ‘Satr’ are of two types: (1) Rulings of ‘Satr’ for men and women during Ṣalāĥ. (2) Rulings of ‘Satr’ outside of Ṣalāĥ, in terms of who can look at certain body-parts of certain individuals. Here is some detail for the first type in question-answer form:
Question: What part of a man is included in his Satr and what are the relevant rulings for him in Ṣalāĥ?
Answer: Ṣadr-ush-Sharī’aĥ, Badr-uṭ-Ṭarīqaĥ, ‘Allāmaĥ Maulānā Muftī Muhammad Amjad ‘Alī A’ẓamī عَـلَيْهِ رَحْـمَةُ الـلّٰـهِ الۡـقَـوِی said: A man’s Satr-e-‘Awrat is from below his navel to below his knees, i.e. covering this area of his body is Farḍ. The navel is not included in Satr, but the knees are. Nowadays, many people wear Taĥband[1] or pyjamas in such a way that a part of their abdomen below their navel remains exposed. If they wear a shirt which covers this area and its skin is not exposed then it is fine, otherwise, Ḥarām. If a quarter (of the area under the navel) remains exposed during Ṣalāĥ, then Ṣalāĥ will be invalid. Some audacious people expose their knees and even thighs in front of other people, this is Ḥarām too, and anyone who is habitual of this is Fāsiq [sinner]. (Ibid, pp. 481)