Reference: Silsilah Fataawa Noor ‘alad-Darb – Tape 2
Question: [He] was ill and they did surgery on him; [as a result] he missed a number of obligatory prayers, so is he required to pray them all together once he has recovered, or should he pray all these obligatory prayers at their appointed times, such as [making-up] the [missed] ‘asr [prayer] at the time of the ‘asr prayer, and the [missed] zhuhr [prayer] at the time of the zhuhr prayer, and the [missed] maghrib [prayer] at the time of the maghrib prayer, and so on?
Response: He is [required] to pray them all together at the same time; because when the Prophet ﷺ missed the ‘asr prayer during the Battle of Khandaq, he made it up before the [time of] maghrib, and likewise it is obligatory upon everyone who has missed a prayer to make it up all together [at one time], and not to delay it.
Question: And in reality, many people are troubled by this [situation] and ask about it, as they have been made to understand that the [missed] prayer is to be prayer at its time; [so] the [missed] zhuhr [prayer is to be made-up] at the [time of the next] zhuhr [prayer], and in doing so, they rest the entire day without praying.
Response: And this is a mistake, because the time the obligatory prayers are to be performed is when they are remembered; this is based on the statement of the Prophet sal-Allaahu ‘alayhi wa salam:
«If one of you sleeps and misses a prayer, or forgets it, let him offer the prayer when he remembers» – Saheeh Muslim