What is the ruling on someone who forgot to perform ghusl (ritual bathing) after Fajr prayer, then led people in Dhuhr prayer and remembered during the prayer, but completed his prayer out of embarrassment, and what are the implications for him and for the congregants, and how does he expiate this sin?

If the Imam remembers that he is in a state of major ritual impurity (junub) during the prayer, he must appoint someone else to complete the prayer. He must also repent and repeat the prayer that he performed while in a state of major ritual impurity. Scholars have differed regarding the validity of the congregants' prayer behind an Imam who is in a state of ritual impurity. The majority of scholars hold that it is not valid, while others – such as Ibn Uthaymeen – consider it valid as long as the congregants are unaware of the Imam's ritual impurity.