Whoever prays forgetting that he does not have wudu has to repeat the prayer because the Messenger of Allah (peace and blessings be upon him) said: “Allah does not accept any prayer if a person breaks his wudu, until he does wudu.”

If you still have wudu, and nothing has invalidated your wudu, then you do not have to repeat your wudu. Prophet Muhammad (peace and blessings be upon him) prayed all five prayers with one wudu on the day when he conquered Makkah.