There is no specific account of this event in the 6th-century writings of Gildas. The story is known from the Historia Brittonum, attributed to the Welsh historian Nennius, which was a compilation in Latin of various older materials (some of which were historical and others mythic or legendary) put together during the early 9th century, and surviving in 9th-century manuscripts – i.e., some 400 years after the supposed events. According to John Morris's textual analysis of the Historia, this tale derived from a north Welsh narrative which was mainly about Emrys (Ambrosius Aurelianus), which the compiler of the Historia incorporated into a framework drawn from a Kentish chronicle, together with details from a Life of Saint Germanus.
This is a literal translation of the Latin from the L. Faral (Paris 1929) edition of the text (sections in square brackets supplied from T. Mommsen's 1892 edition):
It happened however after the death of Vortimer, son of King Vortigern, and after the return of Hengist with his forces, they called for a false Council, so that they might work sorrow to Vortigern with his army. For they sent legates to ask for peace, that there might be perpetual friendship between them. So Vortigern himself with the elders by birth of his people opinion was with them all, that they should make peace, and their legates went back and afterwards called together the conference, so that on either side the Britons and Saxons (Brittones et Saxones) should come together as one without arms, so that friendship should be sealed.
And Hengistus ordered the whole of his household that each one should hide his knife (artavum) under his foot in the middle of his shoe. 'And when I shall call out to you and say "Eu nimet saxas" (Hey, draw your swords!), then draw your knives (cultellos) from the soles of your shoes, and fall upon them, and stand strongly against them. And do not kill their king, but seize him for the sake of my daughter whom I gave to him in matrimony, because it is better for us that he should be ransomed from our hands.' And they brought together the conference, and the Saxons, speaking in a friendly way, meanwhile were thinking in a wolvish way, and sociably they sat down man beside man (i.e. Saxon beside Briton). Hengistus, as he had said, spoke out, and all the three hundred elders of King Vortigern were slaughtered, and only he was imprisoned, and was chained, and he gave to them many regions for the ransom of his soul (i.e. life), that is Est Saxum, Sut saxum
Read more about this topic: Night Of The Long Knives (Arthurian)