On joining a congregational prayer, I realized that I was not in a state of ablution (Wudu). What should one do: offer salaam and break Raka'h and do ablution and return or wait till the congregation is over and repeat the prayer?

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

If a person begins the prayer and then remembers that he is not in a state of ablution or that his ablution was invalidated during the prayer, then he must stop the prayer. It is not permissible for him to continue it without ablution.

The Prophet said: "Allah does not accept the prayer of one of you whose ablution was nullified until he performs ablution again". [Al-Bukhaari and Muslim]

Allah knows best.