As quoted in Vita Divi Iuli [The Life of the deified Julius] (121 CE) by Suetonius, paragraph 33 (Caesar: ... "Iacta alea est", inquit. – Caesar said ... "the die is cast".)
Said when crossing the river Rubicon with his legions on 10 January, 49 BC, thus beginning the civil war with the forces of Pompey. The Rubicon river was the boundary of Gaul, the province Caesar had the authority to keep his army in. By crossing the river, he had committed an invasion of Italy.
A contrasting account from Plutarch, Life of Pompey, 60.2.9: