In the name of Allah, the Most Merciful, Bestower of Mercy.
Question: What is the ruling if, after finishing the Prayer (Salāh), a person remembers or notices that he didn’t wash one of his limbs, or realises that he didn’t wipe (masah) over his socks―does he repeat the wudū and the Prayer, or is his Prayer sound?
Answer: If he is sure that he did not wash some limb that is obligatory to wash and then he prayed in that state of wudū, then it is obligatory upon him to repeat the wudū and repeat the Prayer which he prayed.
Source: Fath Ar-Rabb Al-Wudūd Fī Fatāwā war-Rasā’il war-Rudūd 1/181, of Shaikh Ahmad Ibn Yahyā An-Najmī (may Allah’s mercy be upon him).
Additional note: The same applies to a person who realises after the prayer has finished that he is not in a state of wudū―or during the Prayer he remembers or realises that he doesn’t have wudū, he must stop the Prayer, go and perform wudū, and then start the Prayer again from the beginning.