Sibelius - Andante Festivo
It began as a string quartet, but Sibelius' Andante Festivo eventually became a symphonic masterpiece.
Sibelius originally scored this rousing music for string quartet in 1922. Featuring the composer's impassioned string writing, the unmistakable hummable tune was made all the more luscious when he rescored the piece for string orchestra and timpani in 1938. It's a remarkable work, combining a rich sonority with a sense of poignance — not to mention majesty.
He's better known for his symphonic works like Finlandia, but the fact that this particular piece began its life as a string quartet shows that some of his biggest ideas had the smallest origins.