Is it permissible to pray while lying on one's back and perform dry ablution (tayammum) for some prayers, under the pretext that it is easier and that performing wudu by going to the tap is difficult, even though one is sometimes able to perform wudu?

A sick person must use water for purification unless there is a valid excuse preventing it. If he is unable to perform ablution by himself, someone else should perform it for him; otherwise, he should perform tayammum. So, whoever finds water and is able to use it, must use it, and it is not permissible for him to perform tayammum except for an excuse, based on the saying of the Prophet, peace and blessings be upon him: "Pure earth is the ablution of a Muslim as long as he does not find water." If using water causes severe hardship, delays healing, or increases the illness, then tayammum is permissible. For ease, it is permissible for the sick person to combine prayers with a single ablution.