According to George Sand’s daughter Solange, who stayed with the composer at the monastery in Majorca when the preludes were written, “My mother gave a title to each of Chopin’s wonderful Preludes; these titles have been preserved on a score he gave to us.” That titled score is lost. But Solange did record the names of the preludes, apparently without assigning the names to the prelude numbers.
 
This is the famous "Raindrop" prelude.