How does a prisoner pray in a dark underground cell, knowing nothing about the prayer times due to the darkness?

If one is imprisoned in a dark place and does not know the prayer times with certainty, he must imitate someone who informs him based on observation, such as the prison guard. Otherwise, he must exert his own effort (ijtihad) and should not imitate another mujtahid unless he is unable to perform ijtihad himself.

In case of doubt about the time due to clouds or imprisonment, one must perform ijtihad and deduce the time using studies (of celestial bodies) and recitations (of dhikr, etc.). A blind person performs ijtihad like a sighted one, because he shares these same signs. Ijtihad is required if a trustworthy person does not inform of the time's entry based on observation; but if he informs based on observation, then his information must be acted upon.

If a trustworthy person informs based on ijtihad, it is not permissible for a sighted person capable of ijtihad to imitate him. However, a blind person and one unable to perform ijtihad are permitted to imitate him.

Whoever prays without ijtihad when it is obligatory for him, must repeat the prayer, even if it happened to coincide with the correct time. If he assumed the time had entered and prayed based on assumption without any sign, his prayer is not valid due to his negligence in abandoning ijtihad and signs.

If nothing becomes preponderant in his opinion after performing ijtihad, he must wait until he assumes the time has entered. The cautious approach is to delay the prayer until he is certain the time has entered or preponderantly assumes it has.

If he prays based on ijtihad and the situation does not become clear, there is nothing upon him. If it becomes clear that the prayer occurred within the time or after it, his prayer is sufficient.

If a trustworthy person informs him that his prayer occurred before the time, then if the information is based on knowledge and observation, repetition is obligatory. But if it is based on ijtihad, there is no repetition.