It is called the Declamation of Sallust against Cicero, and bears intrinsic evidence that it was written in after years.

It suited some one to forge pretended invectives between Sallust and Cicero, and is chiefly noteworthy here because it gives to Dio Cassius a foundation for the hardest of hard words he said against the orator.[24] Dio Cassius was a Greek who wrote in the reign of Alexander Severus, more than two centuries and a half after the death of Cicero, and he no doubt speaks evil enough of our hero.

What was the special cause of jealousy on his part cannot probably be now known, but the nature of his hatred may be gathered from the passage in the note, which is so foul-mouthed that it can be only inserted under the veil of his own language.[25] Among other absurdities Dio Cassius says of Cicero that in his latter days he put away a gay young wife, forty years younger than himself, in order that he might enjoy without disturbance the company of another lady who was nearly as much older than himself as his wife was younger.
Now I ask, having brought forward so strong a testimony, not, I will say, as to the character of the man, but of the estimation in which he was held by those who came shortly after him in his own country; having shown, as I profess that I have shown, that his name was always treated with singular dignity and respect, not only by the lovers of the old Republic but by the minions of the Empire; having found that no charge was ever made against him either for insincerity or cowardice or dishonesty by those who dealt commonly with his name, am I not justified in saying that they who have in later days accused him should have shown their authority?
Their authority they have always found in his own words.

It is on his own evidence against himself that they have depended--on his own evidence, or occasionally on their own surmises.
When we are told of his cowardice, because those human vacillations of his, humane as well as human, have been laid bare to us as they came quivering out of his bosom on to his fingers! He is a coward to the critics because they have written without giving themselves time to feel the true meaning of his own words.

If we had only known his acts and not his words--how he stood up against the judges at the trial of Verres, with what courage he encountered the responsibility of his doings at the time of Catiline, how he joined Pompey in Macedonia from a sense of sheer duty, how he defied Antony when to defy Antony was probable death--then we should not call him a coward! It is out of his own mouth that he is condemned.