Firstly:
If someone forgets and does not wash one of the parts that are to be washed in wudu, or did not wipe it if it is one of the parts that must ordinarily be wiped, such as the head, or should be wiped in exceptional circumstances, such as a plaster cast, his wudu is not valid and he must go back and do wudu properly. The evidence for that is the report narrated by Muslim (359) from `Umar ibn al-Khattab (may Allah be pleased with him), who said: A man did wudu and omitted a place the size of a fingernail on his foot. The Prophet (blessings and peace of Allah be upon him) saw him and said: “Go back and do your wudu properly,” so he went back, then he prayed.
An-Nawawi (may Allah have mercy on him) said in his commentary on this hadith: This hadith indicates that whoever omits a small part of that which must be purified, his purification is not valid, and this is a matter on which the scholars are agreed… This hadith also indicates that if someone omits anything of the parts that are to be washed when purifying oneself by mistake, his purification is not valid.(Sharh Muslim 3/123).
He must also make sure to wash the parts one after another (without lengthy interruption). The guideline on that is if the part dries up and the traces of water disappear. The scholars of the Permanent Committee said: If someone forgets to wash one of the parts that are to be washed when doing wudu, or part of it, even a small part, then if he is still doing wudu or has just finished, and the traces of water are still there on his limbs and have not dried up, then he may wash only what he forgot and what comes after that.
But if he realizes that he forgot to wash one of the limbs that must be washed in wudu, or part of it, after it has dried up, or during the prayer, or after he has offered the prayer, then he must start wudu all over again and do it as Allah has prescribed, and he must repeat the prayer in full, because the actions were not done one after another in this instance.(Fatawa al-lajnah ad-Da’imah, Vol. 2, 6/92).
Secondly:
What the imam should have done is exit the prayer and appoint someone to lead the worshippers in completing the prayer, then go and do wudu in the proper sequence, wiping over his arm if he could not wash it or there was a plaster cast on it, then wipe his head and wash his feet, because doing the actions in the correct order, one straight after another, is one of the obligatory parts of wudu.
If it could be harmful to pass his wet hands over his arm or the plaster cast – which is very unlikely – then he should do tayammum for the omitted part before or after doing wudu, as he wishes. That is done by striking the ground with his hands, then wiping his face and hands with them, not what this imam did of wiping the part itself, because tayammum is prescribed to be done in a specific manner, and Allah did not instruct us to wipe the five parts of wudu with dust; rather He said: {and wipe over your faces and your hands} [an-Nisa’ 4:43].
Shaykh Ibn Baz (may Allah have mercy on him) was asked about someone who did wudu when he had a wound on his hand that water should not get onto, and he forgot to do tayammum for it, and he prayed. He replied:
If there is an injury in one of the parts that water should reach when doing wudu, that cannot be washed or wiped over because that would lead to exacerbating the injury or delaying recovery, then what this person must do is tayammum. So if someone does wudu, omitting the part where the injury is, and begins to pray, then whilst he is praying he remembers that he did not do tayammum, he should do tayammum and start his prayer all over again, because what he did of his prayer before doing tayammum was not valid…(Majmu` Fatawa ash-Shaykh Ibn Baz 10 /197).
Based on that, the imam must repeat the prayer, because impurity cannot be removed unless one does wudu completely and properly. As he did the prayer like that, he actually prayed when in a state of minor impurity, and Allah does not accept any prayer offered without purification.
Thirdly:
If the one who is praying behind an imam realizes that his imam is not in a state of purity during the prayer, he must leave the prayer with the imam and complete the prayer on his own. If he did not realise that his imam was not in a state of purity until after the prayer was complete, then his prayer is valid.
Shaykh Ibn `Uthaymin was asked about someone who prayed behind an imam, and he noticed a spot on the imam’s foot the size of a dirham that the water had not reached. What should this worshipper do when he is praying behind that imam, and notices that dry spot?
He replied:
He should intend to continue the prayer on his own, then when he has completed the prayer, he should tell the imam. As for those worshippers who were not aware of that, their prayer is valid, but this one member of the congregation should leave the imam and complete the prayer on his own, because now he is praying behind an imam whose prayer he believes to be invalid, because he did not do wudu properly. Therefore he should exit the prayer and say the taslim, then when the imam says the taslim, he should inform him about that and that it is obligatory for the imam to repeat his wudu and prayer. As for the other worshippers, nothing is required of them.(Al-Liqa’ ash-Shahri, no. 40).
Based on that, the prayer of the worshippers – in the incident asked about – is subject to further discussion:
As for those who realized during the prayer that the imam was in a state of minor impurity, as he had not done wudu properly, their prayer is invalid and they must repeat it, because they prayed behind an imam knowing that he was not in a state of purity.
As for those who did not realize what the imam had done, or did not know why the imam did that (wiping his hands on the wall) whilst he was praying, they do not have to do anything and their prayer is valid.
And Allah knows best.
