The Prelude Op. 28, No. 15, by Frédéric Chopin, known as the “Raindrop” prelude, is one of the 24 Chopin preludes. Usually lasting between five and seven minutes, this is the longest of the preludes. The prelude is noted for its repeating A♭, which appears throughout the piece and sounds like raindrops to many listeners.

https://en.m.wikipedia.org/wiki/Prelude,_Op._28,_No._15_(Chopin)