[Haskell-cafe] dumb monad syntax question

I've been reading Phil Wadler's monad papers from the early '90s, and it's been interesting to see how the monad concept evolved over the course of those years. But I haven't been able to track down the first use of the "do" notation for monads. Can anyone tell me where that came from? I'd appreciate paper citations if it was presented initially in a paper. Thanks in advance, Mike