A group among the ‘Ulama have stated that it is not jaa-iz for a person performing salaah in a sitting posture to stand up – then perform the ruku’and sajdah. The majority of the ‘ulama including the four a-immah say it is jaa-iz (permissible) to do so. Likewise it is also jaa-iz to stand and perform these. This applies only to nafl salaah. It is not permissible for the able-bodied who can stand, to sit and perform the fard salaah.