What is the ruling on a sick person praying on their back when capable of praying on their side?

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The apparent view from Ahmad is that the prayer is valid if performed on the back, as it constitutes a form of orientation (towards the Qibla), similar to how the deceased is oriented toward the Qibla. However, the evidence suggests it is invalid because it contradicts the command of the Prophet (peace be upon him) which specifies praying "on his side." Furthermore, the allowance for lying on the back is restricted to the case where one is incapable of praying on the side, suggesting it is impermissible if lying on the side is possible, as it involves abandoning the requirement of facing the Qibla while capable of fulfilling it.