Tacitus, Annals 4.
When Gaius Asinius and Gaius Antistius were consuls [23 CE], Tiberius enjoyed the ninth year of a stable republic and happiness at home–since he considered Germanicus’ death a blessing. Fortune unexpectedly began to unsettle affairs and Tiberius either became savage himself or was complicit in the savagery of others.
The beginning and the cause were Aelius Sejanus, the prefect of the praetorians. I spoke earlier of his power. Now I will explain where he came from, his character, and the crime he used to seize an empire.”
Asinio C. Antistio consulibus nonus Tiberio annus erat compositae rei publicae, florentis domus (nam Germanici mortem inter prospera ducebat), cum repente turbare fortuna coepit, saevire ipse aut saevientibus viris praebere. Initium et causa penes Aelium Seianum cohortibus praetoriis praefectum, cuius de potentia supra memoravi: nunc originem, mores, et quo facinore dominationem raptum ierit, expediam.