Ruling on abandoning standing during Salah without an excuse

A: Standing up during Salah is a Rukn (essential pillar) without which, Salah is invalid because of Allah’s Saying: And stand before Allâh with obedience [and do not speak to others during the Salât (prayer)]. , and the Prophet’s saying when asked about the way a sick person may offer Salah: “Offer Salah standing and if you cannot, then sitting; and if you cannot, then lying on your side.” Those who offer obligatory Salah sitting without an excuse, their Salah is invalid because they deliberately ignore a Rukn of Salah. As for voluntary Salah, it can be offered sitting even without an excuse and one shall have half the reward of those who offer it standing.May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.