Dio, Book II. "Uttering many strange reproaches publicly, as had been agreed upon, against his father, whom he charged with being a tyrant and a breaker of treaties."
Zonaras 7, 10.
He also joined battle with the people of Gabii and fared ill in the conflict, but overcame them by a ruse; for he suggested to his son Sextus that he desert to their side. And that there might be some plausible pretext for his desertion, Sextus reproached his father publicly as a tyrant and a breaker of treaties, and the latter flogged his son and put himself on the defensive. Then, according to arrangement, the son made his pretended desertion to the people of Gabii, taking along with him money and companions. The enemy believed the trick both on account of the cruelty of Tarquin and because at this time also the son spoke many words of truth in abusing his father and by his conduct seemed to have become thoroughly estranged from him. So they were very glad to receive him, and in his company made many incursions into Roman territory and did it no slight damage. For this reason, and because he privately gave some of them money and also spent it lavishly for public purposes, he was chosen general by them and was entrusted with the management of their government. Thereupon, sending a man secretly, he acquainted his father with what had occurred, and asked him for a plan regarding the future. The king made no answer to the emissary, in order that he might not, in case he were recognized, either willingly
