Nocturne No. 20 en Do Dièse Mineur, Op. posth.

Chopin wrote this nocturne in 1830 for his sister Ludwika and never published it; it appeared twenty-six years after his death, which is what the op. posth. means.

It is the piece Władysław Szpilman plays in The Pianist, and it carries that association now whether or not you want it to.