What is the ruling on praying while sitting, claiming exhaustion?

Scholars unanimously agree that standing in obligatory prayer is a duty for the one who is able. However, it is permissible for the sick and those unable to stand to pray while sitting, based on the Prophet's (peace and blessings be upon him) saying to Imran ibn Husayn: "Pray standing; if you are unable, then sitting; and if you are unable, then on your side."

Exhaustion is not an excuse permitting one to pray while sitting, unless the prayer time is running out and the exhausted person is genuinely unable to stand, such that standing would cause unbearable hardship, or there is a fear of harm like falling, etc. In such a case, there is no blame upon him for praying while sitting, and he would be excused.