Is it permissible to pray sitting on a chair in a public place when there is no suitable space to pray standing, and what are the conditions and method for performing it, and is it obligatory to repeat it later?

Prayer holds a great status in Islam; it is the second pillar after the two testimonies, and it is the first act of a servant that will be examined. Severe warnings have been established for those who are negligent with it or abandon it. Allah says: "Then succeeded them a generation who neglected prayers and pursued desires; so they are going to meet evil." And Allah says: "So woe to those who pray, [but] who are heedless of their prayer." It is obligatory to maintain performing prayers at their prescribed times, and it is not permissible to delay them under the pretext of not finding an isolated place. Rather, they should be performed in public places, even in a classroom. Prayer must be performed standing for those who are able, according to the Prophet's (peace be upon him) saying: "Pray standing; if you are unable, then sitting; and if you are unable, then on your side." Obligatory prayer performed sitting is not valid for one who is able to stand, because standing is a pillar that is not waived as long as one is able to perform it. Likewise, bowing (ruku') and prostration (sujud) are two pillars without which the prayer is not valid for one who is able; for one who is unable, gesturing for them suffices. Performing prayer on a chair while sitting is not permissible except for one who is unable to stand.