How can a patient who has undergone surgery purify himself from major ritual impurity (al-hadath al-akbar) and pray, when he is unable to perform ghusl (ritual bath) and his condition has persisted for a week?

Allah Almighty made tayammum a substitute for water for those who cannot use it due to illness or travel, as He says: (And if you are ill or on a journey or one of you comes from the place of relieving himself or you have contacted women and find no water, then seek clean earth and wipe over your faces and your hands [with it]. Indeed, Allah is ever Pardoning and Forgiving). Based on this, whoever has undergone surgery and is unable to perform ghusl or wudu, must perform tayammum.