Forgot to Do Wudu and Prayed: Should I Repeat the Prayer?

Whoever prays forgetting that he does not have wudu has to repeat the prayer, because the Messenger (peace and blessings of Allah be upon him) said: “Allah does not accept any prayer if a person breaks his wudu , until he does wudu.” (Narrated by al-Bukhari from the hadith of Abu Hurayrah (may Allah be pleased with him) in Kitab al-Wudu)

This is in contrast to the person who has some impurity (najasah) on his clothes and he forgets it. He does not have to repeat his prayer because Jibril came to the Prophet (peace and blessings of Allah be upon him) whilst he was praying, and told him that there was something dirty on his shoes . He took them off and continued praying. This was narrated by Ahmad from the hadith of Abu Sa’id al-Khudri (may Allah be pleased with him) in his Musnad. 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. 

And Allah knows best.