M. Tullius Cicero In Catilinam 3.10

[10] But they said there had been this dispute between Cethegus and the rest,—that Lentulus and others thought it best that the massacre should take place and the city be burnt at the Saturnalia, but that Cethegus thought it too long to wait. 5
  And, not to detain you, O Romans, we ordered the letters to be brought forward which were said to have been given them by each of the men. First I showed his seal to Cethegus; he recognised it: we cut the thread; we read the letter. It was written with his own hand: that he would do for the senate and people of the Allobroges what he had promised their ambassadors; and that he begged them also to do what their ambassadors had arranged. Then Cethegus, who a little before had made answer about the swords and daggers which had been found in his house, and had said that he had always been fond of fine arms, being stricken down and dejected at the reading of his letters, convicted by his own conscience, became suddenly silent. Statilius, being introduced, owned his handwriting and his seal. His letters were read, of nearly the same tenor: he confessed it. Then I showed Lentulus his letters, and asked him whether he recognised the seal? He nodded assent. But it is, said I, a well-known seal;—the likeness of your grandfather, a most illustrious man, who greatly loved his country and his fellow-citizens; and it even though silent, ought to have called you back from such wickedness.