What did Ambedkar say about a state built on one religion?
He opposed it, and did so in strong terms in his book on Partition, where he argued that a state organised around the supremacy of one religion would be a danger to liberty regardless of which religion it was. He was writing there against the idea of a theocratic Pakistan, but the objection he stated was general: rule in the name of a faith puts the believers of that faith above their fellow citizens and turns everyone else into a subject.
That principle is built into the Constitution he chaired. It gives the state no religion, guarantees freedom of conscience to all, and makes equal citizenship independent of faith. He did not arrive at this as an outsider to religion — he was a deeply religious man who ended his life a Buddhist — but as someone who held that the state's business is justice among citizens, not the enforcement of any creed.
It is worth being exact about the scope of the claim: he was analysing the two-nation theory of the 1940s, not commenting on later Indian politics he did not live to see. What can be said without stretch is that a state ranking citizens by religion is the arrangement his life's work was aimed against.