What is Cinna instructed to leave for Brutus?

Cinna is instructed to leave letters for Brutus. In Julius Caesar, at the end of Act 1, Scene3, Cassius sends letters to Brutus by way of Cinna. Cassius altered these letters to make them seem as if they were written by Roman citizens who do not want Caesar to use his power to govern Rome. The letters prove to be the catalyst that drives Brutus to forsake his friendship with Caesar.

Cinna is instructed to leave Letters for Brutus