Caesar did not himself report in any of his works that he uttered these famous words. But Suetonius does:
While he was thus hesitating, the following incident occurred. A person remarkable for his noble mien and graceful aspect, appeared close at hand, sitting and playing upon a pipe. When, not only the shepherds, but a number of soldiers also flocked from their posts to listen to him, and some trumpeters among them, he snatched a trumpet from one of them, ran to the river with it, and sounding the advance with a piercing blast, crossed to the other side. Upon this, Caesar exclaimed, " Let us go where the omens of the Gods and the iniquity of our enemies call us. The die is now cast."
cunctanti ostentum tale factum est. quidam eximia magnitudine et forma in proximo sedens repente apparuit harundine canens; ad quem audiendum cum praeter pastores plurimi etiam ex stationibus milites concurrissent interque eos et aeneatores, rapta ab uno tuba prosiliuit ad flumen et ingenti spiritu classicum exorsus pertendit ad alteram ripam. tunc Caesar: 'eatur,' inquit, 'quo deorum ostenta et inimicorum iniquitas uocat. iacta alea est,' inquit.