Praise be to Allah.
Standing is an essential part of the prayer, and if a person does not stand during the prayer, from the initial takbeer to the final salaam, and has no valid excuse for that, his prayer is invalid. Allaah says (interpretation of the meaning): “And stand before Allaah with obedience” [al-Baqarah 2:238].
The rule that standing is an essential part of the prayer applies especially to the fard (obligatory) prayers; it is not obligatory to stand in the naafil prayers, in which sitting is permitted, but the one who sits will have half of the reward of one who stands.
The evidence that this rule applies only to the fard prayers is the hadeeth in which the Prophet (peace and blessings of Allaah be upon him) said: “Pray standing.” (al-Bukhaari, 1066).
The Prophet (peace and blessings of Allaah be upon him) used to pray naafil prayers (sitting) on a saddle, but when he wanted to pray a fard prayer, he would get down off his saddle (al-Bukhaari, 955; Muslim, 700), so that he could fulfil the essential conditions of the prayer, i.e., standing and facing the qiblah.
If a person prays a naafil prayer sitting, when he is able to stand, he will have half of the reward of one who stands. ‘Abd-Allaah ibn ‘Umar narrated that he said to the Messenger of Allaah (peace and blessings of Allaah be upon him), “I was told, O Messenger of Allaah, that you said that the prayer of one who prays sitting down is equal to half of the prayer of one who prays standing, but you pray sitting down.” He said, “That is right, but I am not like one of you.” (From the hadeeth narrated by Muslim, 735).
Al-Nawawi said, commenting on this hadeeth: this hadeeth is to be understood as meaning that a naafil prayer done sitting down when one is able to stand will bring half of the reward of one who prays standing. But if a person prays a naafil prayer sitting down because he is unable to stand, his reward will not be reduced, and he will have the same reward as one who prays standing. With regard to Fard prayers, if a person prays sitting down when he is able to stand, his prayer will not be valid and he will not get any reward for it, in fact this is counted as a sin. (Sharh Muslim. 6/258)
So we say to those who sit on the chairs and do not stand during the fard prayers: it is not permissible for you to sit on your chairs when you are able to stand, unless it would be so difficult for you as to cause harm, but a slight difficulty is no excuse.