It is not permissible for you to pray sitting down in obligatory prayers unless you are unable to stand, or standing is very difficult for you, or you are sick and there is the fear that your sickness may become worse if you pray standing up. It is permissible to pray sitting down in optional prayers, even without an excuse, but the reward of the one who sits in that case is half the reward of one who stands

If the sick person cannot do Wudu with water because he is unable to or because he is afraid that it will make his sickness worse or delay his recovery, then he should do Tayammum. If the sick people cannot stand, then he may pray sitting. If he is unable to pray sitting, he may pray lying on his side. If he is unable to pray on his side, he may pray lying on his back with his feet towards the Qiblah.

If the worshipper is able to stand and lie down, but he is not able to sit, then he may pray standing, leaning forward for bowing and prostrating, and reciting the Tashahhud whilst standing, then saying the Taslim. If there is a sick person who can either stand for the entire prayer or sit for the entire prayer, then he should pray sitting.