When Paganini first met Berlioz after one of Berlioz's concert, Paganini had acquired a superb viola, a Stradivarius  — "But I have no suitable music. Would you like to write a solo for viola? You are the only one I can trust for this task," Paganini said. Berlioz thus began writing a solo for viola, but the 'solo' turned out to be a piece involved the orchestra in such a way as not to reduce the effectiveness of the orchestral contribution. When Paganini saw the sketch of the allegro movement, with all the rests in the viola part, he told Berlioz it would not do: he expected to be playing continuously. They then parted, with Paganini disappointed.

Wikipedia: "Harold en Italie"

Youtube: 1st part