Is it obligatory to repeat the prayer if the worshipper remembers after finishing it that he was in a state of ritual impurity?

Your prayer is invalid and you must repeat it, as purity is a condition for the validity of prayer. Whoever prays while in a state of impurity and then realizes it, must purify himself and repeat the prayer. Ibn Taymiyyah said: "Whoever forgets ritual purity (taharat al-hadath) and prays forgetfully, must repeat the prayer with purity, without dispute."