When it is obligatory to repeat the prayer
30-5-2016 | IslamWeb
Question:
Assalaamu alaykum wa rahmatullaah. I asked a question yesterday regarding wiping over the socks during Wudu' (fatwa 2614667), and I thank you so much for the very swift response. It was a referral to another fatwa that covered half of the question. The other half of the question was not clearly addressed, and I feel that I probably should have been slightly clearer in posing it. For example, I make Wudu', and go to pray the Thuhr. After the azan of the 'Asr, I realize that, for example, I did not perform Wudu' properly for the Thuhr, or that there was some impurity on my clothing, or that there was something else that would have made my prayer invalid. My question is: must I repeatn the Thuhr, or is that not necessary? I came across a source saying that it would not be necessary to repeat the prayer if such a thing was noticed after the time for that prayer had passed but that if I noticed it before the time for prayer had passed, I must repeat it. I cannot recall if the source was authentic or not, and as such would like a clarification. Thank you so much for your help, and may Allaah reward you.
Answer:
All perfect praise be to Allaah, The Lord of the worlds. I testify that there is none worthy of worship except Allaah and that Muhammad 
is His slave and Messenger.
There is a difference between not making Wudhoo' or leaving out one of its obligations and pillars, and praying with the presence of some impurity on the clothes. If a person prays and forgot to make Wudhoo' or forgot to wash some of the body parts that must be washed in Wudhoo' in a way that makes his Wudhoo' invalid, then if he remembers afterwards, even after having prayed, he must repeat the Wudhoo' as well as the prayer that he had made with that Wudhoo'.
However, in the case of a person who had prayed and then found some impurity on his clothes or his body which he did not know of, or which he knew of but forgot, then his prayer is valid according to the most preponderant opinion of scholars.
This is because avoiding impurity is part of abandoning what is forbidden, and this is forgiven, whereas not doing what is ordered is not forgiven, as has been clarified by the scholars. Shaykh ibn ‘Uthaymeen 
clarified this issue as he said:
“The most preponderant opinion is that if a person prays with impurity because he was unaware of its presence or forgot its presence, then his prayer is valid, such as if his garment is stained with impurity and he did not wash it right away and then he prayed while forgetting that it was there, then his prayer is valid, as Allaah says (what means): {Our Lord, do not impose blame upon us if we have forgotten or erred.} [Quran 2:286].
The same thing applies if he was unaware of it and did not know about it until after he had prayed, then his prayer is also valid for the previous verse.
As regards the prayer of a person who prays without Wudhoo' out of forgetfulness, then he must repeat the prayer, such as if his Wudhoo' is invalidated and the prayer time comes and he prayed without Wudhoo', forgetting that his Wudhoo' was invalidated, and then he remembers later, then he must repeat the prayer after performing Wudhoo'.
Also, if he is invited to a banquet and he eats meat and does not know what meat it is, and he prays and then discovers after he prayed that it was camel meat, then he must perform Wudhoo' and repeat the prayer even if he did not know when he ate it that it was camel meat. Hence, the one who prayed while forgetting that he was without Wudhoo' must repeat the prayer, and the one who prayed while forgetting that he had some impurity (on his body or clothes) or who did not know that he was in such a state is not obliged to repeat the prayer. The difference between the two is that in regard with the issue of not being in a state of Wudhoo', it constitutes abandoning an order, and abandoning an order out of forgetfulness or out of ignorance exempts one from sin, but it does not exempt him from repeating the prayer or any other act of worship, in a correct and valid manner, as he is able to do this and make up for the deficiency.
As regards the one who prays in an impure garment out of forgetfulness or ignorance, then this is like doing what is forbidden. Doing what is forbidden out of forgetfulness or ignorance exempts him from sin because of his forgetfulness or ignorance, and this means that he did not do what is forbidden. Hence, this exemption from sin because of forgetfulness or ignorance renders his prayer valid, as if he did not do what is forbidden (i.e. as nothing affected the validity of his prayer at all).”
Allaah knows best.