How to correct the prayer depends on what the mistake is (major or minor), why it happens (intentionally, accidentally), the nature of the prayer (obligatory or recommended) and who makes it (the individual praying alone, the one leading the prayer or the one following an imam).
This answer will assume one is praying an obligatory (or supererogatory) prayer alone and the act was unintentional.
Mistakes such as those which violate any of the prerequisites (shurut) invalidates the prayers, violating the pillars (rukn) or mandatory aspects of prayers (wajibat) can invalidate the prayer and need to be compensated for via sujood al-sahu. Others are mistakes that do not invalidate the prayers so do not require repetition.
The prayer is built upon a number of pillars - should any pillar be omitted or incorrectly performed, the entire prayer is invalidated.
The following are the pillars of prayers:
The obligatory parts of prayer, of which there are eight, are as follows :
Then there are prerequisites to the prayer - adult, Muslim, sane, wudu, facing qibla, intention.
To make up for a mistake, there are two extra sujoods (sujood al-sahu) which are done towards the end of prayer (both obligatory or non-obligatory) when you have forgotten an essential act just before or after the tasleem. Whilst some of the schools of thought state say one prostration is done for forgetfulness, the ahadith all suggest the messenger undertook two prostrations.
If you missed a precondition like not covering awrah, making intent, or facing qibla you will need to repeat the prayer.
For examples of minor mistakes where the prayer is valid and sujood al-sahu are not required:
The evidences for this comprise the following:
Allah's Messenger (saw) stood up for the zuhr prayer and he should have sat (after the second rak'ah but he stood up for the third rak'ah without sitting for Tashahhud) and when he finished the prayer he performed two prostrations and said takbir on each prostration while sitting, before ending (the prayer) with taslim; and the people too performed the two prostrations with him instead of the sitting he forgot.” (Sahih al-Bukhari 1230)
The Prophet (saw) offered the noon prayer and he gave the salutation at the end of two rak'ahs. He was asked. Has the prayer been shortened? then he offered two rak'ahs of the prayer and made two prostrations (at the end of it). (Abu Dawud 1014)
Once Allah's Messenger (saw) offered five rak'ah in the Zuhr prayer, and somebody asked him whether there was some increase in the prayer. Allah's Messenger (S.A.W.) said, "What is that?" He said, "You have offered five rak'ah." So Allah's Messenger (saw) performed two prostrations of Sahu after tasleem. (Sahih al-Bukhari 1226)
The Prophet (saw) said: "If one of you does not know whether he prayed three or four (rak'ahs), let him pray a rak'ah then prostrate twice after that when he is sitting. Then if he prayed five (rak'ahs), they (the two prostrations) will make his prayer even-numbered, and if he had prayed four, they will annoy and humiliate the Shaitan.”’ (Nasai 1239)
All Mistakes Invalidate the Prayer
Sujood as-Sahw is Only for Omissions
Intentional Errors Can Be Corrected
Recitation Errors Don't Affect Prayer
Sujood as-Sahw Complicates the Prayer
To correct a minor mistake in or error in prayer (excluding omitting a prerequisite), there are two extra sujoods (sujood al-sahu) which are done towards the end of prayer (both obligatory or non-obligatory).
What should I do if I make a mistake in my Salat?
If the mistake is minor, such as forgetting a rakah or unintentionally adding an extra one, you can perform Sujood as-Sahw at the end of your prayer to rectify the mistake.
How do I perform Sujood as-Sahw?
To perform Sujood as-Sahw, you should make two prostrations before or after the Tasleem (saying "Assalamu Alaikum wa Rahmatullah" to end the prayer). The specific timing depends on the nature of the mistake.
Does forgetting a non-obligatory part of the prayer require Sujood as-Sahw?
No, Sujood as-Sahw is generally not required for missing non-obligatory (Sunnah) parts of the prayer, as it is specifically for rectifying mistakes in obligatory parts.
Can I correct a mistake if I realize it after finishing the prayer?
If you realize a mistake after completing the prayer and it's a major error, you may need to repeat the prayer. For minor errors realized afterwards, Sujood as-Sahw can be performed if the time is still close to the original prayer.
Are there any mistakes in Salat that cannot be corrected with Sujood as-Sahw?
Intentional mistakes or significant unintentional errors in essential components of the prayer, like the recitation of Al-Fatiha, may require repeating the prayer, as Sujood as-Sahw is intended for minor and unintentional mistakes only.
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