Purification is a condition for the validity of Prayer. Wudu’ should be made with pure water. Allah says in the Qur’an, “O you who believe, when you rise for prayer, wash your faces and your hands up to the elbows and lightly rub your heads and (wash) your feet up to the ankles.” (Al-Mai’dah 6).
Sheikh Ibn `Uthaymeen, a prominent Saudi Muslim scholar (may Allah shower mercy on his soul), states that: “Whoever prays forgetting that he did not have wudu’ has to repeat the Prayer. Abu Hurayrah (may Allah be pleased with him) reported that the Messenger (peace and blessings be upon him) said, “Allah does not accept any Prayer if a person breaks his wudu’, until he does wudu’” (Al-Bukhari). This is in contrast to the person who has some impurity (najasah) on his clothes and forgets it. He does not have to repeat his Prayer. Abu Sa`id Al-Khudri (may Allah be pleased with him) reported that Jibreel came to the Prophet (peace and blessings of Allah be upon him) while he was praying and told him that there was something dirty on his shoes. He took them off and continued praying (Ahmad). This hadith proves that the one who is unaware of the najasah does not have to repeat his Prayer, and this applies also to the one who forgets that there is some najasah present.”