Perhaps ironically, it is Brutus himself who speaks to this idea. After he and Cassius have been defeated by Antony's forces, Brutus chooses to commit suicide rather than endure capture. As he runs on his own sword, held firmly in place by Strato at Brutus' request, Brutus speaks his dying words:
Caesar, now be still.
I killed not thee with half so good a will.
The words "be still" suggests that Caesar's ghost, which had appeared to Brutus before the final battle, can now rest. Caesar's murder has been avenged.