What is the ruling on performing tayammum when water is present?

Question:
What is the ruling on one who performs tayammum while water is available? What is the ruling on his prayer while he is in this state for a long time? Does this act make him a disbeliever?

Answer:
One who performs tayammum while water is available is not permitted to do so; his prayer is invalid. Allah says: [Al-Ma'idah: 6]. If he is performing tayammum while he is healthy—not ill—and is able to use the water, and the water is available, then his prayer is invalid, and his ruling is the ruling of one who abandons prayer. If he is knowledgeable and aware, not ignorant, he has committed disbelief—may Allah keep us safe from that. But if he is ignorant, he is to be taught; because some people may be ignorant of this.
So the conclusion is that he is not permitted to perform tayammum while water is available and he is able to use it.
As for if he is ill and water would harm him, or if water is not available, then he performs tayammum, as Allah the Exalted said: [Al-Ma'idah: 6]. The sick person for whom water is harmful is of the same category as the one who has no water.
So if he prayed without water while he was ignorant: if it was a small amount, he should pray it and repeat it; and if it was a large amount, we hope that it is excused for him due to his ignorance. Because when the Messenger of Allah (peace and blessings be upon him) taught the one who was deficient in his prayer—and he was performing it like pecking—he ordered him to repeat it, but he did not order him to repeat the prayers that had passed, which his condition indicated he had performed in that manner. He only intended for him to repeat the current prayer, due to his ignorance and lack of understanding.