Dr. Muzammil Siddiqi, former president of the Islamic Society of North America (ISNA) states: “Performing prayer while sitting on a chair or without any regard for qiblah is not allowed for people who have the ability to perform the prayer in a normal way, paying due regard to all the postures of prayer (such as qiyam, ruku`, sujud, etc.).
Also, seeking the proper direction of the qiblah is also a condition for the validity of prayers. The sick and travelers, either by road, air or by sea are, of course, allowed to pray in the sitting posture, while lowering their heads a little bit for ruku` and more for sujud.
Similarly sick people are allowed to pray in whatever position they deem convenient. For others, whether they are praying in private places or public, all the postures of prayer are necessary.”
Shedding more light on the issue of performing prayer while sitting on a chair, the prominent Saudi Islamic lecturer and author, Sheikh Muhammad Saleh Al-Munajjid points out: “Standing is an essential part of the prayer, and if a person fails to stand during the prayer, from the initial takbeer to the final salam, with no valid excuse for that, his prayer is invalid.
Allah Almighty says: “And stand before Allah with obedience” (Al-Baqarah :238)
This rule applies especially to the obligatory prayers; i.e., in performing supererogatory prayers, sitting is permitted, though it earns one half of the reward of performing prayer in standing. The evidence that this rule applies only to the obligatory prayers is the hadith in which the Prophet (peace and blessings be upon him) said: “Pray while standing.” (Reported by Al-Bukhari)
The Prophet (peace and blessings be upon him) used to pray supererogatory prayers (sitting) on a saddle, but on getting to pray an obligatory prayer, he would get down off his saddle so that he could fulfill the essential conditions of the prayer, i.e., standing and facing the qiblah. (Reported by Al Bukhari and Muslim)
If a person prays supererogatory prayer sitting, when he is able to stand, he will have half of the reward of one who stands. ‘Abdullah Ibn ‘Umar narrated that he said to the Messenger of Allah (peace and blessings be upon him) “I was told, O Messenger of Allah, that you said that the prayer of one who prays while sitting down is half of the prayer performed by one standing, but you pray sitting down.” He said, “That is right, but I am not like anyone of you.” (Reported by Muslim)
In his commentary on the aforementioned hadith, Imam An-Nawawi said: “This hadith is to be understood as meaning that supererogatory 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 supererogatory 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 obligatory 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 this is actually counted as a sin.”
So, we say to those who perform prayers while sitting on the chairs during the obligatory prayers that it is not permissible for them to sit on chairs when they are able to stand, unless they find it extremely difficult to stand; but a slight difficulty is not an excuse.”