Question: In which cases does one have to repeat salat?
ANSWER
Some of them are as follows:

[To carry out a salat within its prescribed time is called adaa. To carry it out after its prescribed time has ended is called qada (making it up). To carry it out again in order to make up for a defect or an error is called i’aadah (repeating).]

1. If one omits one of the wajib elements of salat deliberately, it will be wajib upon one to repeat that salat. If one omits one of the wajib elements of salat because one has forgotten to do it, then one will have to do sajda as-sahw (prostration of forgetfulness) at the end of salat.

2. If one doubts whether one has performed the current fard salat or not, one must repeat it if the time for it has not expired.

3. If one does not do sajda as-sahw deliberately though one has done something that necessitates it, it will be wajib to repeat that salat.

4. After one has completed salat, if an adil Muslim says that one performed it wrongly, it will be better for one to repeat it. If two ‘adil Muslims say so, it will be wajib to repeat it. If the person saying it is not ‘adil, one does not have to believe him.

[An ‘adil Muslim means a Sunni Muslim who avoids major sins and who does not habitually commit minor sins.]

5. If one, while taking a ghusl, forgets to wash a part of one’s body and then performs salat but remembers afterwards that one did not wash it, one must wash that part and repeat the fard salat. In the Maliki Madhhab, if one remembers it even a month later, one must wash only that part immediately. If one does not wash it immediately, one’s ghusl becomes null and void. However, the salats one performed before it are counted as valid.

6. If one who does not know the location of the qibla has tried to determine it but has not performed salat in the direction that one has decided to be right, one has to perform it again even if one understands that one has found the right direction by chance. If one starts salat without making an effort to determine the direction of the qibla and then finds out that one found the right direction, one must repeat the salat, anyway. If one has tried to locate the qibla and performed salat in the direction that one has decided to be right, one does not have to repeat salat even if one finds out later that it was not the right direction.

7. If one notices something that breaks the wudu’ of the imam, one must repeat the salat that one performed behind him.

8. If the imam remembers in salat that he is not in a state of wudu’ or if something nullifying his salat happens during salat, he must immediately inform the jama’at about it. If he notices it after the salat, he tells or sends somebody to inform the ones he remembers to have joined the jama’at. The ones who learn it have to repeat the salat. Those who do not hear it will be pardoned. According to another qawl, the imam does not have to inform the jama’at. The salats of the jama’at are considered valid, but the imam has to repeat his salat.

9. If a sahib-i tartib (a person who does not have any missed salat or has less than six) has a missed (qada) salat to make up, (s)he must perform the missed salat first and then perform the salat of the current time. If the remaining time before the end of the time for the current salat is so little that it permits the performance of only it, (s)he must perform the current salat first and then perform the qada salat. However, if a person who performed the current salat because (s)he misjudged that there was very little time left finds out later that there is enough time, (s)he must perform the qada salat first and then repeat the current salat.

10. If one omits ta’dil-i arkan deliberately in salat, it will be wajib upon one to repeat that salat.

11. If a prisoner who is fastened by the hands and feet cannot perform tayammum, without wudu’ (s)he does ruku’ and sajda without reciting anything in salat. If (s)he cannot do this, either, (s)he must perform salat standing through gestures (ishaarah). When (s)he is free, (s)he must perform all these salats again.

12. If one in prison cannot find a clean place, water, or soil to perform salat, one should pretend to be performing salat without reciting anything. One must repeat these salats when one is released from prison.

13. If one who is on a bus operating between cities falls asleep and experiences a nocturnal emission during sleep, one should perform salat by doing tayammum if it is not possible to get off the bus. One must repeat that salat after taking a ghusl.

14. In boarding schools or in similar places, if bathrooms are opened after the sun rises, one can perform salat by doing tayammum if it is not possible to take a ghusl somewhere else. One must repeat that salat after taking a ghusl.

15. If a person who has an ‘adil friend with him/her performs tayammum without asking about water and starts to perform salat and then is told that there is water, (s)he must do wudu’ and repeat the salat.

16. If one who thinks one’s water has run out sees after the salat that one does have water, one repeats the salat that one performed with tayammum. Likewise, when a person who (thinks that one is in a state of wudu’ although one does not and thereby) performs a salat without wudu’ remembers afterwards that one did not have wudu’ (when one performed that salat), in this case one must repeat the salat.

17. If water is close but there is an enemy, a wild or poisonous animal, a fire near the water or if one is imprisoned, one does tayammum instead of wudu’ and performs salat. But since these are caused by creatures, one has to perform the salat again after taking a ghusl and wudu’.

18. If one performs salat thinking that the time for it has not started yet, one must repeat it even if one finds out after salat that one performed it within its prescribed time. One has to start salat knowing for sure that the time for it has started.

19. If there is no secluded place, one does not open one’s private parts near others to clean them after relieving oneself. One waits until the others leave the place. If the time of salat becomes short one does not clean one’s private parts near others, and one does not wash one’s underpants, either. One performs salat with impurity on, for it is more blessed to abstain from the haram than to do the fard. Later on when one finds a secluded place, one cleans one’s private parts, washes one’s underpants, and performs the salat again. (Se’adet-i Ebediyye)

Question: What are the cases that necessitate repetition of salat?
ANSWER
If one omits an obligatory (fard) element of salat, it is fard to repeat that salat. If one deliberately omits a wajib element of salat, it is wajib to repeat that salat. If one omits a wajib element because one has forgotten to do it, one must do sajda as-sahw at the end of salat. For example, if one omits ta’dil-i arkan deliberately, it is wajib to repeat that salat. If any makruh act is committed, except for omitting a wajib element, or a sunnat element is omitted in salat, then it is sunnat, not wajib, to repeat that salat.

It is permissible to break salat in order to get rid of the urge to relieve oneself or to break wind.