How does a prisoner whose hands and feet are tied behind his back pray if the guards refuse to untie him at the time of prayer?

Prayer does not cease to be obligatory for a person as long as they are conscious and rational. They should pray according to their ability: standing, or sitting, or on their side, or lying down indicating with gestures. If they are unable to gesture, they should perform the actions of prayer in their heart. They are also excused from ritual purity if they are unable to achieve it, and they should pray with tayammum (dry ablution) if they are unable to perform ablution with water themselves or have someone perform it for them. If they are unable to perform tayammum, they should pray in their current state.