What is the ruling on praying while in a state of major ritual impurity due to forgetfulness?

Question : If a man sat for the Fajr prayer because he was unable to use water, and then when the time for Zuhr arrived, he forgot that he was in a state of major ritual impurity, and he performed the ablution for each prayer, he prayed Zuhr, Maghrib, and 'Isha', and 'Asr, Maghrib, and 'Isha, and then he remembered.

Answer : It is obligatory for him to perform the full ritual bath and repeat the four prayers.