Praise be to Allah.
It is well known and no Muslim has any excuse for not knowing that purification from impurity, whether minor or major, is obligatory and is a condition of prayer being valid. Whoever prays without purifying himself, whether deliberately or by mistake, his prayer is invalid and he has to repeat it; if it is done deliberately then he has committed a major sin.
Shaykh al-Islam Ibn Taymiyah (may Allah have mercy on him) said:
The Muslim should not pray facing any direction other than the qiblah, or without doing wudoo’, bowing and prostrating. Whoever does that deserves to be criticized and punished. End quote.
Manhaaj al-Sunnah al-Nabawiyyah, 5/204
There is a stern warning issued to one who does that:
It was narrated from ‘Abd-Allah ibn Mas’ood (may Allah be pleased with him) that the Prophet (peace and blessings of Allah be upon him) said: “It was ordered that one of the slaves of Allah be flogged in his grave with one hundred lashes, but he kept asking until it was reduced to a single blow. So he was given a single blow, then his grave was filled with fire. When he came round, he said: ‘Why did you flog me?’ It was said to him: ‘You offered one prayer without being taahir, and you passed by one who was being wronged and did not help him.’” Narrated by al-Tahhaawi in Mushkil al-Athaar, 4/231; classed as hasan by al-Albaani in al-Silsilah al-Saheehah, 2774.
The scholars are agreed that whoever prays without being taahir because he believes that it is permissible or out of disrespect is a kaafir. He should be asked to repent, and if he repents, all well and good, otherwise he should be executed.
But if he prays without wudoo’ out of negligence, not because he thinks it is permissible or out of disrespect, Imam Abu Haneefah (may Allah have mercy on him) is of the view that this is also kufr, but the majority of scholars are of the view that he is not a kaafir, rather he has committed a major sin.
Al-Nawawi (may Allah have mercy on him) said:
If he knows that he is impure and that it is haraam to pray when one is impure, then he has committed a grave sin, but he does not become a kaafir thereby in our view, unless he regards that as permissible. But Abu Haneefah said: He has committed kufr because of his disrespect.
Our evidence is that it is a sin so it is akin to zina and the like. End quote.
Al-Majmoo’ (2/84); Rawdat al-Taalibeen, 10/67
See the Hanafi view in al-Bahr al-Raa’iq, 1/302, 151; 5/132; Haashiyat Ibn ‘Aabideen, 3/719
The one who has prayed without purifying himself has to repent and seek forgiveness, and resolve not to do such a thing again, then he should repeat the prayer that he offered without purifying himself. Allah accepts the repentance of those who repent to Him. And he does not have to enter Islam anew.
And Allah knows best.
See also question no. 27091.