Which is better in prayer for one who is able to sit: praying on a chair or on the ground, knowing that he can prostrate when sitting on the ground?

Standing in the obligatory prayer is a pillar without which the prayer is invalid, and the worshipper does not transition to praying while sitting except after becoming unable to stand. So, whoever is unable to stand is permitted to pray while sitting, but this does not justify abandoning prostration, for it is a pillar without which the prayer is invalid. As for supererogatory prayers, it is permissible for one who is able to stand to pray them while sitting, and he receives half the reward of one who prays standing. He must prostrate on the ground if he is able. The preferred sitting posture in place of standing in supererogatory prayers is iftirash (sitting on the left foot with the right foot upright) or tarabbu' (cross-legged). Some have also said tawarruk (sitting on the left thigh with both feet extended to the right) or setting up the right knee while sitting on the left. If someone is unclear about the legal evidence regarding a matter, they should ask the people of knowledge.