Not sure if it is the original but the source where I first saw this rain effect was on ShaderToy - The Drive Home by Martijn Steinrucken aka BigWings[1]
The accompanying video tutorial[2] is super interesting too as well as the HN discussion at the time [3].
[1] https://www.shadertoy.com/view/MdfBRX
[2] Link to first part: https://www.youtube.com/watch?v=eKtsY7hYTPg