وَالْقِيَامُ فِي غَيْرِ النَّفْلِ
For one who is able, standing is a pillar in Fard and Wajib prayers. Nafl prayer has a separate concession allowing sitting. The legal boundary of standing is that the person is not bent so far that, if the arms were extended, the hands would reach the knees.
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Prayer Pillars: Standing and Recitation
وَالْقِيَامُ فِي غَيْرِ النَّفْلِ
For one who is able, standing is a pillar in Fard and Wajib prayers. Nafl prayer has a separate concession allowing sitting. The legal boundary of standing is that the person is not bent so far that, if the arms were extended, the hands would reach the knees.