We had a thread similar to this a few months back. From What did people think clouds were before we could scientifically describe them?, /u/RomeosDistress quotes Augustine of Hippo (354-430 CE):

The clouds, according to the testimony of those who have walked through them in the mountains, have this vaporous appearance, formed, as they are, of the most minute drops which are gathered and rolled together. And if further condensation takes place, so that one large drop is formed out of many small ones, the air, unable to support it, yields to its weight as it travels, down, and this is the explanation of rain. (The Literal Meaning of Genesis (De Genesi ad Litteram), Volume 1, Book Two, Chapter 4)

That gives a terminus ante quem for associating rain, clouds, and water vapor. The earliest proposed date in that thread is before 600 BC (The Iliad).